Index: lucene/src/test/org/apache/lucene/search/TestMultiPhraseQuery.java
===================================================================
--- lucene/src/test/org/apache/lucene/search/TestMultiPhraseQuery.java	(revision 1062325)
+++ lucene/src/test/org/apache/lucene/search/TestMultiPhraseQuery.java	(working copy)
@@ -22,6 +22,7 @@
 import org.apache.lucene.index.TermsEnum;
 import org.apache.lucene.index.IndexReader;
 import org.apache.lucene.index.MultiFields;
+import org.apache.lucene.search.Explanation.IDFExplanation;
 import org.apache.lucene.store.Directory;
 import org.apache.lucene.util.BytesRef;
 import org.apache.lucene.document.Document;
@@ -30,6 +31,7 @@
 import org.apache.lucene.util.LuceneTestCase;
 
 import java.io.IOException;
+import java.util.Collection;
 import java.util.LinkedList;
 
 /**
@@ -285,4 +287,44 @@
     new MultiPhraseQuery().toString();
   }
   
+  public void testCustomIDF() throws Exception {
+    Directory indexStore = newDirectory();
+    RandomIndexWriter writer = new RandomIndexWriter(random, indexStore);
+    add("This is a test", "object", writer);
+    add("a note", "note", writer);
+    
+    IndexReader reader = writer.getReader();
+    IndexSearcher searcher = new IndexSearcher(reader);
+    searcher.setSimilarity(new DefaultSimilarity() {
+      
+      @Override
+      public IDFExplanation idfExplain(Collection<Term> terms,
+          IndexSearcher searcher) throws IOException {
+        return new IDFExplanation() {
+
+          @Override
+          public float getIdf() {
+            return 10f;
+          }
+
+          @Override
+          public String explain() {
+            return "just a test";
+          }
+          
+        };
+      }   
+    });
+    
+    MultiPhraseQuery query = new MultiPhraseQuery();
+    query.add(new Term[] { new Term("body", "this"), new Term("body", "that") });
+    query.add(new Term("body", "is"));
+    Weight weight = query.createWeight(searcher);
+    assertEquals(10f * 10f, weight.sumOfSquaredWeights(), 0.001f);
+
+    writer.close();
+    searcher.close();
+    reader.close();
+    indexStore.close();
+  }
 }
Index: lucene/src/java/org/apache/lucene/search/MultiPhraseQuery.java
===================================================================
--- lucene/src/java/org/apache/lucene/search/MultiPhraseQuery.java	(revision 1061618)
+++ lucene/src/java/org/apache/lucene/search/MultiPhraseQuery.java	(working copy)
@@ -25,6 +25,7 @@
 import org.apache.lucene.index.Term;
 import org.apache.lucene.index.DocsEnum;
 import org.apache.lucene.index.DocsAndPositionsEnum;
+import org.apache.lucene.search.Explanation.IDFExplanation;
 import org.apache.lucene.util.ArrayUtil;
 import org.apache.lucene.util.BytesRef;
 import org.apache.lucene.util.ToStringUtils;
@@ -131,6 +132,7 @@
   private class MultiPhraseWeight extends Weight {
     private Similarity similarity;
     private float value;
+    private final IDFExplanation idfExp;
     private float idf;
     private float queryNorm;
     private float queryWeight;
@@ -140,12 +142,14 @@
       this.similarity = searcher.getSimilarity();
 
       // compute idf
-      final int maxDoc = searcher.maxDoc();
+      ArrayList<Term> allTerms = new ArrayList<Term>();
       for(final Term[] terms: termArrays) {
         for (Term term: terms) {
-          idf += this.similarity.idf(searcher.docFreq(term), maxDoc);
+          allTerms.add(term);
         }
       }
+      idfExp = similarity.idfExplain(allTerms, searcher);
+      idf = idfExp.getIdf();
     }
 
     @Override
@@ -238,7 +242,7 @@
       ComplexExplanation result = new ComplexExplanation();
       result.setDescription("weight("+getQuery()+" in "+doc+"), product of:");
 
-      Explanation idfExpl = new Explanation(idf, "idf("+getQuery()+")");
+      Explanation idfExpl = new Explanation(idf, "idf(" + field + ":" + idfExp.explain() +")");
 
       // explain query weight
       Explanation queryExpl = new Explanation();