The thermal decomposition of a sample of a commercial composite solid propellant binder, hydroxy-terminated polybutadiene (HTPB), was studied by thermogravimetry-derivative thermogravimetry-differential scanning calorimetry simultaneous techniques (TG-DTG-DSC) applied simultaneously at heating rates varying from 8 to 60° C min −1. Decomposition of HTPB under this dynamic heating program is indicated by two major stages of weight loss of different natures and indistinct separation. The first stage is primarily depolymerization, cyclization and crosslinking of material not undergoing depolymerization. The second stage is decomposition of a residue yielded in the first stage. Kinetic parameters of the decomposition process at different heating rates were evaluated by means of the maximum rate method for a TG trace by Fouss et al., the simplified method for the DSC thermogram by Maycock, and the multiple-heating rate method for TG traces by Freidman. The DSC data indicate that in the temperature range of the first stage of decomposition, the overall process is also exothermic even at the lower temperature, and that the exothermicity is reduced with increasing heating rate. The apparent activation energy for the decomposition, calculated by the simplified DSC method, increases with increasing heating rate. Increasing the heating rate results in increased depolymerization in the first stage, and increased temperatures of the exothermic DSC peak and completion of the second stage. Increasing the sample size leads to reduced depolymerization in the first stage and increased residue in the second stage. The relevance of the generated kinetic data to solid propellant combustion is compared.