This contribution considers to what extent two molecular parameters, the glass transition temperature and the cohesive energy density, relate to the yield behavior of epoxy-based glassy thermosets. Eight different formulations consisting of four aliphatic and four aromatic resins with varying molecular weights between crosslinks were investigated over a broad range of test temperatures. Additionally, one aromatic formulation is studied over a range of stress states and test temperatures. The results indicate that both the glass transition and cohesive energy density are governing parameters that relate to the yield response of these systems. A model is proposed to incorporate these parameters and to predict the yield response as a function of strain rate, temperature, and stress state. The functional form of the results also indicate that the activation energy density (i.e., the activation energy divided by the activation volume) may be the material characteristic that relates to yield of these systems rather than each term individually. (C) 2004 Wiley Periodicals, Inc.