Novel biocidal organic N-halamines (based on imidazoline dione rings) were used as a substituents for poly(p-methyl styrene). The biocidal polymers and substituents have been investigated using thermogravimetric analysis. The thermal resistance of investigated compounds was compared to those of non-halogenated precursors. The introduction of chlorine atoms to polymers decreases their thermal resistance comparing to precursors but efficient stabilization is possible by using octyl tin mercaptide. The complex mechanism of thermal decomposition of polymers has been discussed. (C) 2014 Elsevier B.V. All rights reserved.