In the present study the thermal behavior of phosphorus- and phosphorus/nitrogen-containing functional polymers was studied. The polymers were synthesized via oxidative chlorophosphorylation reaction of butadiene rubber and were subsequently subjected to hydrolysis, aminolysis and/or alcoholysis in order to introduce appropriate functionalities. The successful modifications of the polymer and presences of the respective functional groups were determined using Fourier transform infrared spectroscopy. It was found that the product of hydrolysis of the modified butadiene rubber contains acidic groups in its structure while the products of aminolysis and alcoholysis contain both acidic and amine and/or alcohol-derived moieties. The kinetic analyses of the thermal decomposition reaction were evaluated using thermogravimetric analysis and subsequently Friedman and Ozawa–Flynn–Wall methods revealing three-stage degradation process. Calorimetric measurements were performed for the studied polymers, and mass spectrometric analyses were used to identify the decomposition products for phosphorus-containing polymer. The overall results allowed to reveal the influence of the attached phosphorus- and phosphorus/nitrogen-containing functional groups to the thermal degradation of the cross-linked polymers.