Abstract: A process for the preparation of a hard foam containing urethane groups or containing urethane and isocyanurate groups involves reactinga) an organic and/or modified organic polyisocyanate withb) at least one high-molecular-weight compound containing at least two reactive hydrogen atoms and, if desired,c) a low-molecular-weight chain extender and/or cross-linking agent,in the presence ofd) a blowing agent,e) a catalyst and, if desired,f) assistants and/or additives,where the blowing agent (d) used is at least one organic carboxylic acid, preferably an aliphatic monocarboxylic acid, alone or in combination with water and/or a physical blowing agent, and the catalyst (e) used is a polyisocyanurate catalyst, preferably an alkali metal salt or ammonium salt of an organic carboxylic acid or tris(dialkylaminoalkyl)-s-hexahydrotriazine.