The instant invention provides an isocyanate trimerization catalyst system, a precursor formulation, a process for trimerizing isocyanates, rigid foams made therefrom, and a process for making such foams. The trimerization catalyst system comprises: (a) a phosphatrane cation; and (b) an isocyanate-trimer inducing anion; wherein said trimerization catalyst system has a trimerization activation temperature in the range of equal to or less than 73° C. The precursor formulation comprises (1) at least 25 percent by weight of polyol, based on the weight of the precursor formulation; (2) less than 15 percent by weight of a trimerization catalyst system, based on the weight of the precursor formulation, comprising; (a) a phosphatrane cation; and (c) an isocyanate-trimer inducing anion; wherein said trimerization catalyst system has a trimerization activation temperature in the range of equal to or less than 73° C.; and (4) optionally one or more surfactants, one or more flame retardants, water, one or more antioxidants, one or more auxiliary blowing agents, one or more urethane catalysts, one or more auxiliary trimerization catalysts, or combinations thereof. The process for trimerization of isocyanates comprises the steps of: (1) providing one or more monomers selected from the group consisting of an isocyanate, a diisocyanate, a triisocyanate, oligomeric isocyanate, a salt of any thereof, and a mixture of any thereof; (2) providing a trimerization catalyst system comprising; (a) an phosphatrane cation; and (b) an isocyanate-trimer inducing anion; (c) wherein said trimerization catalyst system has a trimerization activation temperature in the range of equal to or less than 73° C.; (3) trimerizing said one or more monomers in the presence of said trimerization catalyst; (4) thereby forming an isocyanurate ring. The process for making the PIR foam comprises the steps of: (1) providing one or more monomers selected from the group consisting of an isocyanate, a diisocyanate, a triisocyanate, oligomeric isocyanate, a salt of any thereof, and a mixture of any thereof; (2) providing polyol; (3) providing a trimerization catalyst system comprising; (a) a phosphatrane cation; and (b) an isocyanate-trimer inducing anion; wherein said trimerization catalyst system has a trimerization activation temperature in the range of equal to or less than 73° C.; and (4) optionally providing one or more surfactants, one or more flame retardants, water, one or more antioxidants, one or more auxiliary blowing agents, one or more urethane catalysts, one or more auxiliary trimerization catalysts, or combinations thereof; (5) contacting said one or more monomers, and said polyol, and optionally said one or more surfactants, and optionally said one or more flame retardants, and optionally said water, and optionally said one or more antioxidants, and optionally said one or more auxiliary blowing agents in the presence of said trimerization catalyst system and optionally said one or more urethane catalysts, and optionally said one or more auxiliary trimerization catalysts; (6) thereby forming said polyisocyanurate/polyurethane rigid foam. The PIR foam comprises the reaction product of one or more monomers selected from the group consisting of an isocyanate, a diisocyanate, a triisocyanate, oligomeric isocyanate, a salt of any thereof, and a mixture of any thereof with polyol in the presence of a trimerization catalyst system comprising a phosphatrane cation, and an isocyanate-trimer inducing anion, and optionally one or more surfactants, optionally one or more flame retardants, optionally water, optionally one or more antioxidants, optionally one or more auxiliary blowing agents, optionally one or more additional urethane catalysts, and optionally one or more auxiliary trimerization catalysts, or optionally combinations thereof, wherein the trimerization catalyst system has a trimerization activation temperature in the range of equal to or less than 73° C.The PIR foam comprises the reaction product of one or more monomers selected from the group consisting of an isocyanate, a diisocyanate, a triisocyanate, oligomeric isocyanate, a salt of any thereof, and a mixture of any thereof with polyol in the presence of a trimerization catalyst system comprising a phosphatrane cation, and an isocyanate-trimer inducing anion, and optionally one or more surfactants, optionally one or more flame retardants, optionally water, optionally one or more antioxidants, optionally one or more auxiliary blowing agents, optionally one or more additional polyurethane catalysts, and optionally one or more auxiliary trimerization catalysts, or optionally combinations thereof, wherein the PIR foam has a polyisocyanurate trimer ratio (Abs1410/Abs1595) of at least 5 at a depth of 12 mm from the rising surface of the rigid foam, measured via ATR-FTIR spectroscopy.