The mechanical behavior of compatible glassy polyblends based upon poly (2 , 6-dimethyl-l , 4-phenylene oxide) (PPO) was investigated. In particular, the influence of composition, molecular weight, and molecular weight distribution upon the large deformation tensile properties was assessed. Various possible correlations between the experimentally determined moduli and theory are considered. Included are correlations with density, packing density, composite theory and lattice fluid theory. Similarities in behavior of the compatible glassy polyblends to the phenomenon known as "antiplasticization" is presented. The modeling of the properties of these polymer mixtures via Simplex lattice design is also detailed. Finally, attention is given to the development of compatibility criteria based upon the large deformation tensile property and density measurements. It was shown that composite equations cannot adequately describe the mechanical behavior of compatible PPO based polyblends. However, it is possible to generate a second order Simplex equation which will closely model the modulus-compositional empirical trends. Furthermore, there are strong indications that the interaction term in the Simplex equation can serve as a useful gauge for compatibility and level of compatibility. It was also shown that all the criteria for the phenomenon known as "antiplasticization" were fulfilled by all the compatible PPO based systems examined. For example, the high molecular weight "antiplasticizer" , polystyrene (PS) , when dissolved in PPO, decreases the glass transition temperature of the blend while raising the magnitude of the secant modulus and tensile strength above the value which would be predicted by the rule of mixtures. Packing density was found to be useful for explaining antiplasticization and compatibility. It appears to be the key to understanding the moduli of glassy alloys. The density and packing density are the only equilibrium quantities which pass through a maximum similar to the modulus. These results suggest that compatibility might be handled without resorting to specific molecular interactions