Abstract: A method for producing water-blown molded flexible polyurethane foam which comprises reacting and foaming in one step in a mold a reaction mixture comprising a polyol, an organic polyisocyanate, a urethane catalyst composition and water and a trimerization catalyst which is a quaternary ammonium salt or an alkali metal or alkaline earth metal salt of a Bronsted acid having a pKa of >1, characterized in that the trimerization catalyst is present in an amount from about 0.005 to 0.04 gram milliequivalents per 100 grams of polyol. The preferred trimerization catalyst is an alkali metal N-(2-hydroxyphenyl)methyl glycinate, especially an alkali metal N-(2-hydroxy-5-nonylphenyl)methyl-N-methyl glycinate.