Abstract The intensity of light scattered from dilute solutions of mixtures of two nonidentical polymers in a single solvent was measured. The parameter A24 characterising the thermo‐dynamic interaction of two nonidentical polymer molecules in solution was calculated for several systems polymer 2 ‐ polymer 4 ‐ solvent. For chemically strongly different polymers, A24 is independent of molecular weights of the polymers and probably also of solvent. A24 is related to the compatibility of both polymers and decreases contrary to expectation with increasing chemical difference between polymers 2 and 4.