Interfacial tensions sigma were measured by means of the sessile drop method for mixtures of poly(ethylene oxide) [PEO, M-w = 41 kg/mol] with five samples of poly(dimethylsiloxane)s [PDMS, M-w ranging from 4 to 177 kg/mol] in the temperature interval 70-130 degrees C. Within experimental error sigma does not depend on T and the influences of molar mass can be well described by a linear dependence on M-n(-2/3). The ability of block copolymers to reduce sigma when added in small amounts was studied for two families of these substances. With type I all blocks are made of either DMS or EO and the members differ in the molecular architecture only (diblocks, triblocks, and "bottle-brushes"). Type II comprises diblock copolymers, and the members differ in the chemical composition of the blocks which consist of EO, styrene, or methyl methacrylate. The reduction of sigma upon an increase in x(add), the base mole fraction of the additive in the PEO phase, is substantial for all additives under consideration; it can be well described by an equation of Tang-Huang containing two characteristic parameters : sigma(s), the saturation interfacial tension approached in the limit of large x(add), and x(char), the additive concentration required to achieve Ile of the maximum reduction. In the case of type I block copolymers, upsilon, the total number of DMS segments, turns out to be more decisive for their efficiency than their architecture, in contrast to the theoretical expectation. A lower limit of sigma(s) congruent to 1 mN/m is reached for upsilon > 30. At least the present representatives of type II additives are less efficient than those of type I; an increase in the number of segments does not change sigma(s), in a noteworthy way, whereas there exist indications that it reduces x(char).