Polymers bearing imino sulfonate units in the side chain were prepared by the radical copolymerization of corresponding monomers and photo-assisted thermal crosslinking of the polymers was studied. Although the polymers became insoluble in common organic solvents upon heating, the temperature (T-ins) at which they became insoluble lowered if they were irradiated before heating. The acid generated from the photolysis of imino sulfonate units catalyzed the thermolysis of the remaining imino sulfonate units, causing the crosslinking of the polymer. The T-ins, values were dependent upon the structure of imino sulfonate units and also upon glass-transition temperature of the polymers. A mechanism for the photo-assisted thermal crosslinking of the polymers was discussed.