Improvements in physical, mechanical, and thermal properties of an epoxy network are made with the use of a unique class of molecules that reinforce the network at the molecular scale. These molecules are commonly referred to as antiplasticizers or fortifiers. In this contribution, two types of fortifiers are incorporated into the model epoxy network. One, dimethyl methylphosphonate, is a simple additive while the other, diethyl phosphoramidate, contains a reactive amine and is cured as part of the network. The two approaches provide some unique differences in the physical and mechanical properties of the networks. Several mechanisms of fortification are discussed and correlated to the observed properties. In addition, it is shown that the fortifiers improve the rheological characteristics of the epoxy resin and act as fire-retardants in the cured network. (c) 2006 Wiley Periodicals, Inc.