Poly(methyl methacrylate) (PMMA, M n = 232 000) was used to modify a stoichiometric epoxy (diglycidyl ether of bisphenol A; DGEBA)±anhydride (methyl tetrahydrophthalic anhydride; MTHPA) thermoset. PMMA concentrations in the range 3±7 wt% led to morphologies consisting of a continuous PMMA-rich region that appeared rough and striated in scanning electron micrographs and large domains of the thermoset exhibiting a dispersion of PMMA-rich particles in the micrometre range. These morphologies are the result of the critical point location, estimated at 2.1 wt% PMMA as a result of the high molar mass of the additive. A 5 wt% PMMA led to an increase of the stress intensity factor K IC from 0.65 MPa m 1/2 , for the neat thermoset, to 0.94 MPa m 1/2 . However, T g was reduced from 117 °C for the neat thermoset to about 105 °C for the PMMA-modi®ed material. The T g decrease is ascribed to the differential segregation of both monomers to the PMMA-rich phase. No in¯uence of PMMA addition on the cure kinetics was observed. An upper critical solution temperature was observed, meaning that cloud-point conversions increased with cure temperature. Phase separation took place before gelation in the temperature range investigated in this study.