AbstractThe properties of mixtures of poly(2, 6‐dimethyl p‐phenylene oxide) and poly(styrene) have been measured by DSC, density gradient, dynamic mechanical response, and tensile testing. The mixtures are found to have single glass trasitions that vary continuously with the composition. They also have small negative excess volumes of mixing, indicative of strong polymerpolymer interaction. The dynamic mechanical response of the mixtures shows that the low temperature secondary relaxations are suppressed whereas those at high temperatures are enhanced. These observations imply that mixing on the segmental levc l has occurred. A plot of tensile strength vs composition at different strain rates reveals two regions of failure behavior. The high PS, high strain rate region is brittle, and the high PM2PO, low strain rate region is ductile. When the tensile yield data are treated according to the Ree‐Eyring equation, the addition of PS to PM2PO is found to reduce the flow volum, e of the mixture. The overall effect on mechanical properties of adding small amounts of one component to the other is similar to that of anti‐plasticization. Our experimental observations are consistent with the shifting of the relaxation mechanisms to longer times due to the negative excess volume of mixing.