The influence of some cations from the s and d blocks on the thermal degradation of a polyurethane based on 4,4’-dibenzyl di-isocyanate, poly(ethylene adipate) diol and ethylene glycol was studied. The thermogravimetric data were analysed using the Levi-Reich method. The results obtained were processed on a computer; a mathematical relation and a physical parameter are proposed to characterize the processes.