In this paper, the role of trimerization in polyisocyanurate (PIR) foams and its impact on the physical properties and the fire behavior is discussed. The degree of trimerization was systematically varied using different catalyst concentrations. The enhanced formation of isocyanurate structures through higher catalyst concentrations leads to higher mechanical stability and improved fire behavior as measured by vertical flame spread according to DIN4102 and under forced flaming conditions in the cone calorimeter. A clear correlation between catalyst content, chemical structure of the foams (relative concentration of isocyanurate units representing crosslinking) and fire behavior is found. (C) 2020 Elsevier Ltd. All rights reserved.