1. Measurements of the heats of mixing as between different polymers, which we have carried out, have confirmed the correctness of the assumption that the heat effect plays a decisive part in the process of mutual mixing of polymers. 2. The anomalous behavior of mixtures of SKS-30 with SKB and polystyrene with SKB has been revealed. 3. Experimental data on the heats of mixing of polymers, which has substantial value for theoretical and practical research in the field of polymer mixtures and for the evaluation of possible practical applications of these, has been collected.