A new series of monoepoxide terminated controlled epoxy networks (CENs) and a corresponding soluble fraction polymer (SFP) were prepared to further investigate the effects of chain termination on epoxy thermoset structure-property relationships. CENs having an initial molecular weight between crosslinks (M-c,M-i) of similar to 3000 g/mol using phenylglycidyl ether (PGE) as the chain terminator have thermal and mechanical properties consistent with previously studied monophenol terminated CENs. Glass transition temperature (T-g) decreases monotonically with PGE concentration (epsilon), whereas fracture toughness decreases sharply at a critical PGE concentration (epsilon(c)). A PGE terminated SFP was prepared corresponding to the soluble fraction expected for the CEN composition at epsilon(c). The SFP behaves as a weak antiplasticizer in these epoxy thermosets; T-g is reduced and follows the inverse rule of mixtures, and fracture toughness is slightly reduced. By difference it is inferred that most of the deterioration of epoxy thermoset properties resulting from incorporation of chain terminators above epsilon(c) is a result of the presence of nonelastically active pendant chains and by the increase in M-c. (C) 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 72-79, 2009