The Friedman method is applied to two processes: flexible polyurethane foam (PU) pyrolysis and pine needle combustion, considering two types of runs: dynamic and dynamic + isothermal. In both cases, the TG and DTG runs are taken into account. For both materials, the Friedman method is applied twice: one considering only the dynamic runs and the other considering all runs, dynamic and dynamic + isothermal runs. Different expressions for fitting the variation of the apparent activation energy and the pre-exponential function have been obtained and used to simulate the experimental results. The simulated results are compared with the experimental ones and with those simulated with the n-reaction order models. From the discussion carried out, it has been deduced that the Friedman method is valid when taking into account all the experimental data obtained in different operating conditions and after testing that the simulated results are close to the experimental ones.