diff --git a/Models/PaintRecipe.cs b/Models/PaintRecipe.cs
--- a/Models/PaintRecipe.cs
+++ b/Models/PaintRecipe.cs
@@ -176,8 +176,8 @@ namespace DesertPaintCodex.Models
             Debug.Assert(reactions != null);
             
             // track visited reagents so the reaction is only applied once
-            HashSet<string> reagentSet   = new();
-            List<Reagent>                  prevReagents = new();
+            HashSet<string> reagentSet = new();
+            List<Reagent> prevReagents = new();
 
             foreach (ReagentQuantity ingredient in _recipe)
             {
@@ -360,5 +360,41 @@ namespace DesertPaintCodex.Models
 
             return false;
         }
+
+        public List<(string, string)> MissingReactionPairs()
+        {
+            ReactionSet? reactions = ProfileManager.CurrentProfile?.Reactions;
+            Debug.Assert(reactions != null);
+
+            HashSet<string> reagentSet = new();
+            List<Reagent> prevReagents = new();
+
+            List<(string, string)> missingReactionPairs = new List<(string, string)>(16);
+
+            foreach (ReagentQuantity ingredient in _recipe)
+            {
+                if (reagentSet.Count > 4) return missingReactionPairs; // no more reactions
+
+                string reagentName = ingredient.Name;
+                if (string.IsNullOrEmpty(reagentName)) continue;
+                if (reagentSet.Contains(reagentName)) continue;
+                reagentSet.Add(reagentName);
+
+                Reagent reagent = ReagentService.GetReagent(reagentName);
+
+                // Find reactions.
+                foreach (Reagent prevReagent in prevReagents)
+                {
+                    Reaction? reaction = reactions.Find(prevReagent, reagent);
+                    if (reaction == null)
+                    {
+                        missingReactionPairs.Add((prevReagent.Name, reagentName));
+                    }
+                }
+                prevReagents.Add(reagent);
+            }
+
+            return missingReactionPairs;
+        }
     }
 }