According to a new cluster theory for the concentration-dependence of polymer solution viscosity proposed recently

the concept‘ideal polymer mixture’was re-investigated and defined as one in which there does not exist any net attractive interaction between unlike polymers which could result in inter-chain association between unlike polymers．Based on the new definition

a new viscometric criterion for polymer-polymer miscibility was suggested．Namely

if the overall apparent association constant obtained experimentally (〈Km〉exp) of a mixture solution deviates positively from the weight-averaged value of the two components’ apparent association constants

the mixture could be judged as miscible．It was found that the new criterion agrees well with experimental data．