Equilibrium phase diagrams in composition-conversion space have been calculated for stoichiometric blends of an epoxy resin with a diamine hardener and dendritic hyperbranched polymers, to which various numbers of epoxy groups have been grafted. An attempt has been made to incorporate both the effects of the polydispersity of the resin and the reactivity of the functionalized hyperbranched polymers. The results suggest that below the gel point, the resin-rich regions of the phase diagrams are only weakly affected by reaction of the hyperbranched polymers for any given value of the interaction parameter. However, if the reactivities of all the epoxy groups are comparable, the presence of a highly functionalized modifier is expected to lead to a reduction of the gel point conversion and a narrowing of the composition-conversion window available for cure induced phase separation. The experimental cloud point data are consistent with the model, although they suggest that the functionalized hyperbranched polymers may be significantly less reactive than the resin. Moreover, in the present system, the influence of functionalization on the phase behavior is also strongly linked to the accompanying changes in the interaction parameter.