More formally, for a mix of molecules of A and B, the interactions between unlike neighbors (UAB) and like neighbors UAA and UBB must be of the same average strength, i.e., 2 UAB = UAA + UBB and the longer-range interactions must be nil (or at least indistinguishable).
Namesto tega predpostavimo, da je povprečna jakost interakcij med vsemi molekulami v raztopini enaka.