Young’s modulus, yield (break) strength, and elongation to yield (break) have been measured for blends of poly(2,6‐dimethyl‐1,4‐phenylene oxide) (PPO) with polystyrene (PS), poly(p‐chlorostyrene) (PpClS), and random copolymers of styrene and p‐chlorostyrene (pClS). The significant difference between blend compositions is the compatibility of PPO with each styrene polymer. Blends of PPO with PS or copolymers with 67.1 mole% or less pClS are compatible (i.e., one Tg) and show small synergistic maxima in modulus, strength, and elongation as a function of PPO composition. These maxima correspond to observed maxima in packing density as a result of specific interactions contributing to blend compatibility. A rule of mixtures for one‐phase systems with an adjustable compatibility parameter gives adequate fit to the observed composition dependence of the modulus. In a narrow composition range between 67.8 and 68.6 mole% pClS, copolymers exhibit partial miscibility with PPO. Two mixed composition phases are present. Moduli of these transitional blends follow the same form of synergistic dependence on blend composition as do the compatible blends but strength and elongation exhibit a sigmoidal relation to blend PPO content. At about 20% PPO, strength (and elongation) reaches a minimum as predicted by a simple composite model for a dispersed phase with good adhesion to the matrix. A maximum is reached at ∼80% PPO at which composition blend test specimens yield prior to failure. Blends of PPO with PpClS and with copolymers of ≳68.6 mole% pClS exhibit a broader minimum in strength (and elongation) but a similar maximum at 80% PPO. Unlike the compatible and transitional blends, moduli follow a nonsynergistic composition dependence adequately represented by the series model for two‐phase systems.