N-halamine chemistry has been a research topic of considerable importance in these laboratories for over 2 decades because N-halamine compounds are very useful in preparing biocidal materials. To understand the utility of these compounds, the stabilities and mechanism of halogenation of cyclic N-halamine compounds should be resolved. The important precursor biocidal compound, 2,2,5,5-tetramethylimidazolidin-4-one (TMIO) was considered as a model in this theoretical study. The thermodynamic and kinetic products of monohalogenation were investigated along with tautomerization of TMIO and succinimide theoretically at the level of B3LYP/6-311+G(2d,p). Solvation effects (water and chloroform) were included using the CPCM solvation model with UAKS cavities. Several mechanisms have been proposed for the chlorine migration from the 3-position (kinetic product) to the 1-position (thermodynamic product) of the TMIO ring. The results are in agreement with experimental NMR data.