AbstractAlthough there is general recognition of the value of the Alfrey‐Price relationship, controversy has not abated concerning the theoretical basis for the equations and the accuracy of the monomer reactivity ratios predicted. A heretofore unrecognized possibility for approaching the problem of general monomer reactivity relationships appears to lie in the consideration of terpolymers. The relationship between monomer and terpolymer compositions is given by the equations A more general and yet simplified relationship underlying terpolymers which has not been previously recognized is derived. This relationship is shown to hold for a wide range of monomer types and concentrations. The overall probability of initiating a, c, and b sequences immediately preceded by b, a, and c, respectively, and of undefined terminating units is equal to the probability of terminating a, c, and b sequences with b, a, and c units, respectively, but of undefined initiating units. This relationship of product probabilities is as follows: and is shown to hold for numerous terpolymer and related copolymer systems. It follows that Methods are presented for predicting the behavior of monomers A and C in copolymerization when the behaviors of A‐B and B‐C are known. Furthermore, it is shown that the value of eq. (3) is frequently in the range of 0.02–0.06. This result is probably related to the fact that terpolymerization of monomers of equal reactivity in equimolar concentrations will yield product probabilities equal to 0.037. Variations in individual probabilities require compensating changes in the other related probabilities. Application of this relationship allows the calculation of r13 and r31 where r23, r32, r12, and r21 are known.