diff --git a/src/eckit/container/DenseSet.h b/src/eckit/container/DenseSet.h
index 8fa429f9b..ee570c408 100644
--- a/src/eckit/container/DenseSet.h
+++ b/src/eckit/container/DenseSet.h
@@ -18,6 +18,7 @@
 #include <utility>
 #include <vector>
 
+#include "eckit/log/JSON.h"
 #include "eckit/exception/Exceptions.h"
 
 
@@ -130,6 +131,15 @@ class DenseSet {
         }
     }
 
+    void json(JSON& s) const {
+        s.startList();
+        const_iterator it = cbegin();
+        for (; it != cend(); ++it) {
+            s << *it;
+        }
+        s.endList();
+    }
+
     bool operator==(const DenseSet& rhs) const { return !operator!=(rhs); }
 
     bool operator!=(const DenseSet& rhs) const { return values_ != rhs.values_; }
@@ -139,6 +149,11 @@ class DenseSet {
         return s;
     }
 
+    friend JSON& operator<<(JSON& s, const DenseSet& m) {
+        m.json(s);
+        return s;
+    }
+
     void merge(const DenseSet& other) {
 
         if (other.empty()) return;