Interfacial tensions gamma were measured for mixtures of poly(methylphenylsiloxane) (4 kg/mol) and poly(dimethylsiloxane) (24 kg/mol) in the absence and in the presence of small amounts of the random copolymer poly(dimethylsiloxane-ran-methylphenylsiloxane) (89 mol-% of dimethylsiloxane units, 28 kg/mol) from 25 to 110 degrees C. Approximately 1 wt.-% of the copolymer additive suffices to reduce gamma from ca. 2.2 to 1.6 mN/m. The time dependence of the apparent gamma value in the course of the attainment of equilibria also indicates surface activity. The hypothesis is formulated that the efficiency of the random copolymer for a reduction of gamma is bound to the condition that it is only sparingly soluble in both blend components.