Abstract: A novel catalyst combination for the trimerization of polyisocyanates to polyisocyanurates is disclosed. The catalyst combination comprises (i) a tertiary amine trimerization catalyst, (ii) an alkali metal salt of an N-substituted amide, and (iii) a dibutyl tin di(alkanoate) compound. The use of this catalyst combination in the preparation of polyisocyanurate spray foams enables the systems to be sprayed in either a 1:1 or 2:1 by volume ratio, or ratios between these limits interchangably with no alterations being required in reactant formulations. The polyisocyanurate foams produced using the novel catalyst combination of the invention are characterized by having high resistance to flame and heat distortion.