A novel catalyst combination for the trimerization of polyisocyanates to polyisocyanurates is disclosed. The catalyst combination comprises (i) a tertiary amine trimerization catalyst containing a dimethylamino moiety, (ii) an alkali metal salt of an N-(2-hydroxyphenyl)methyl glycine, and (iii) a monomeric epoxide. The use of these catalysts in the preparation of polyisocyanurate foams results in facile machine preparation of bun stock foam without incurring the processing problems that arise from the prior art method. The polyisocyanurate foams produced using the novel catalyst combination of the invention are characterized by a reduced level of smoke generated on combustion.