diff --git a/library/src/main/java/org/mustangproject/Invoice.java b/library/src/main/java/org/mustangproject/Invoice.java
index 60de0622..006a5875 100644
--- a/library/src/main/java/org/mustangproject/Invoice.java
+++ b/library/src/main/java/org/mustangproject/Invoice.java
@@ -197,7 +197,18 @@ public Invoice setObjectIdentifierReferencedDocument(String id) {
 		setObjectIdentifierReferencedDocument(dr);
 		return this;
 	}
-
+	public Invoice setObjectIdentifierReferencedDocument(String id, String referenceTypeCode) {
+	    ReferencedDocument dr = new ReferencedDocument(id);
+	    dr.setReferenceTypeCode(referenceTypeCode);
+	    return setObjectIdentifierReferencedDocument(dr);
+	}
+	
+	public Invoice setObjectIdentifierReferencedDocument(String id, String referenceTypeCode, Date issueDate) {
+	    ReferencedDocument dr = new ReferencedDocument(id);
+	    dr.setReferenceTypeCode(referenceTypeCode);
+	    dr.setFormattedIssueDateTime(issueDate);
+	    return setObjectIdentifierReferencedDocument(dr);
+	}
 
 	public Invoice setNumber(String number) {
 		this.number = number;
diff --git a/library/src/main/java/org/mustangproject/ZUGFeRD/ZUGFeRD2PullProvider.java b/library/src/main/java/org/mustangproject/ZUGFeRD/ZUGFeRD2PullProvider.java
index d7aab617..04b27466 100644
--- a/library/src/main/java/org/mustangproject/ZUGFeRD/ZUGFeRD2PullProvider.java
+++ b/library/src/main/java/org/mustangproject/ZUGFeRD/ZUGFeRD2PullProvider.java
@@ -662,6 +662,11 @@ public void generateXML(IExportableTransaction trans) {
 			xml += "<ram:AdditionalReferencedDocument>"
 				+ "<ram:IssuerAssignedID>" + XMLTools.encodeXML(trans.getObjectIdentifierReferencedDocument().getIssuerAssignedID()) + "</ram:IssuerAssignedID>"
 				+ "<ram:TypeCode>130</ram:TypeCode>";
+			 // NEW: BT-18-1 scheme identifier
+		    String rtc = trans.getObjectIdentifierReferencedDocument().getReferenceTypeCode();
+		    if (rtc != null && !rtc.isEmpty()) {
+		        xml += "<ram:ReferenceTypeCode>" + XMLTools.encodeXML(rtc) + "</ram:ReferenceTypeCode>";
+		    }
 			if (trans.getObjectIdentifierReferencedDocument().getFormattedIssueDateTime()!=null) {
 				xml += "<ram:FormattedIssueDateTime>" + DATE.qdtFormat(trans.getObjectIdentifierReferencedDocument().getFormattedIssueDateTime()) + "</ram:FormattedIssueDateTime>";
 			}
diff --git a/library/src/test/java/org/mustangproject/ZUGFeRD/BT18ReferenceTypeCodeTest.java b/library/src/test/java/org/mustangproject/ZUGFeRD/BT18ReferenceTypeCodeTest.java
new file mode 100644
index 00000000..2db93307
--- /dev/null
+++ b/library/src/test/java/org/mustangproject/ZUGFeRD/BT18ReferenceTypeCodeTest.java
@@ -0,0 +1,64 @@
+package org.mustangproject.ZUGFeRD;
+
+import junit.framework.TestCase;
+import org.mustangproject.Invoice;
+import org.mustangproject.Item;
+import org.mustangproject.Product;
+import org.mustangproject.ReferencedDocument;
+import org.mustangproject.TradeParty;
+
+import java.math.BigDecimal;
+import java.nio.charset.StandardCharsets;
+import java.util.Date;
+
+import static org.xmlunit.assertj.XmlAssert.assertThat;
+
+public class BT18ReferenceTypeCodeTest extends TestCase {
+
+    /**
+     * Ensures BT-18-1 (scheme identifier) is serialized as ram:ReferenceTypeCode
+     * inside ram:AdditionalReferencedDocument (TypeCode=130).
+     */
+    public void testBT18ReferenceTypeCodeIsWrittenWhenSet() {
+        // Minimal invoice (XRechnung profile => CII output)
+        Invoice i = new Invoice()
+                .setIssueDate(new Date())
+                .setDueDate(new Date())
+                .setDeliveryDate(new Date())
+                .setSender(new TradeParty("Test company", "teststr", "55232", "teststadt", "DE"))
+                .setRecipient(new TradeParty("Franz Müller", "teststr.12", "55232", "Entenhausen", "DE"))
+                .setNumber("INV-123")
+                .addItem(new Item(
+                        new Product("Testprodukt", "", "C62", BigDecimal.ZERO),
+                        new BigDecimal("1.00"),
+                        BigDecimal.ONE
+                ));
+
+        // BT-18: Invoiced Object Identifier (AdditionalReferencedDocument / TypeCode 130)
+        ReferencedDocument rd = new ReferencedDocument("ID unique");
+        rd.setReferenceTypeCode("AJW"); // BT-18-1
+        i.setObjectIdentifierReferencedDocument(rd);
+
+        ZUGFeRD2PullProvider zf2p = new ZUGFeRD2PullProvider();
+        zf2p.setProfile(Profiles.getByName("XRechnung"));
+        zf2p.generateXML(i);
+
+        String xml = new String(zf2p.getXML(), StandardCharsets.UTF_8);
+
+        // Assert: we still have a CII invoice
+        assertTrue(xml.contains("<rsm:CrossIndustryInvoice"));
+
+        // Assert: BT-18 block contains ID, TypeCode=130 and ReferenceTypeCode=AJW
+        assertThat(xml)
+                .valueByXPath("string(//*[local-name()='AdditionalReferencedDocument']/*[local-name()='TypeCode'])")
+                .isEqualTo("130");
+
+        assertThat(xml)
+                .valueByXPath("string(//*[local-name()='AdditionalReferencedDocument']/*[local-name()='IssuerAssignedID'])")
+                .isEqualTo("ID unique");
+
+        assertThat(xml)
+                .valueByXPath("string(//*[local-name()='AdditionalReferencedDocument']/*[local-name()='ReferenceTypeCode'])")
+                .isEqualTo("AJW");
+    }
+}
\ No newline at end of file