The vapour-phase photolysis of isocyanic acid has been studied at 30°C. The results have been explained by a mechanism involving alternative primary dissociations of isocyanic acid : [graphic omitted] I1/I2 increases with decreasing wavelength. Subsequent reactions of these radicals lead to the formation of CO, N2, H2 and solid polymer. The gaseous products were formed approximately in the ratio of 3 : 1 : 1.