A mathematical model is proposed for multifrequency infrared multiphoton dissociation of polyatomic molecules with implications for laser isotope separation. In this model, the dissociation probability is expressed by the functional form based on the product of power-law terms for individual fluences. The data (on the fluence dependence of reaction volume) in the experiments performed on multiphoton dissociation of UF6 cooled at 90 K with multifrequency 16-mum laser beams are utilized to verify our proposed model. This model can show the fluence conditions for high contrast ratios where the dissociation probability for isotopically selective dissociation overwhelms that for nonselective dissociation.