A new class of molecular additives is investigated for epoxy-based crosslinked polymers. These additives are shown to increase modulus and yield stress in the cured networks. In order to elucidate the mechanisms for reinforcement of epoxy thermosets. the effects of additive chemistry and network architecture are considered. Both model and commercial epoxy networks are studied, thereby probing the effect of molecular weight between crosslinks, M-c, on reinforcement. Additionally, the family of phosphates being studied ranges in molecular weight, solubility parameter and density. These parameters are demonstrated to strongly influence the degree of reinforcement and correlate with the ability of the additive to reduce mobility of the polymer network. Mechanisms of reinforcement are discussed. (C) 2004 Society of Plastics Engineers.