The phase behavior of a Poly(2,6-dimethyl-1,4-phenylene ether) (PPE)/diglycidyl ether of bisphenol A type epoxy (DGEBA)/diethyltoluenediamine (DETDA) blend during reaction induced phase separation is predicted using Flory-Huggins theory. DGEBA and DETDA are treated as a single pseudo-component in order to reflect the crosslinking polymerisation that occurs between them, and both the PPE and the DGEBA/DETDA pseudo-component are treated as polydisperse. The Flory-Huggins chi parameter was determined by measuring the extent of reaction at the on-set of phase separation for different compositions and temperatures and comparing the results with theory. The chi parameter is then used to determine the coexistence curves as a function of conversion of the DGEBA/DETDA pseudo-component, from which the extent of reaction at which vitrification occurs is predicted. The model is shown to be in good agreement with experimental results. (C) 2003 Elsevier Science Ltd. All rights reserved.