Phthalonitrile monomers can be polymerized thermally in the presence of small amounts of curing agents into thermosetting polymers. The thermosets exhibit outstanding thermo-oxidative stability, display good mechanical properties, and offer promise as matrices for composite applications. The phthalonitrile cure reaction is typically accomplished with an aromatic diamine, 1,3-bis(8-aminophenoxy) benzene (m-APB), added in the range of 1.5-2% by weight of the monomer in the melt phase. This article addresses the cure reaction with a sulfone-containing diamine, bis[4-(4-aminophenoxy)phenyl] sulfone (p -BAPS), which shows lower volatility as determined from thermogravimetric studies (TGA) compared to m-APB at the processing temperatures typically employed for phthalonitrile cures. Rheometric studies conducted to monitor the viscosity increase during a cure reaction suggest that the cure reaction with m-APB is faster compared to the reaction with B-BAPS. Even though differences are seen in the initial cure rates, the final cured products are similar in terms of the glass transition temperatures and thermal and oxidative stabilities.