Abstract: A novel catalyst combination for the trimerization of polyisocyanates to polyisocyanurates is disclosed. The catalyst combination comprises (i) a tertiary amine trimerization catalyst, and (ii) a quaternary ammonium salt of an alkanoic acid. The use of this catalyst combination in the preparation of polyisocyanurate foams enables the foaming speeds to be varied from relatively slow to fast in the absence of foam shrinkage. The foams produced in accordance with the present invention are useful in applications calling for relatively fast rise characteristics such as in laminate production.