The thermal decomposition mechanism of solids is a main input data of codes in fire research. In this research, the evolution of thermal properties versus temperature is used together with the measurement of gas effluents to understand the decomposition of polyether polyurethane foam (PPUF). A wide characterization of the virgin fuel and the intermediary species that enter into the thermal process showed that a 5-step reaction mechanism is sufficient to describe the decomposition of the foam. The mechanism found is supposed chemically correct by the coupling study of the gaseous emissions. The kinetic constants of each of the 5 reactions are determined with the genetic algorithm method. The mechanism is used in a mathematical model in order to predict the mass loss rate at 4 heating rates and 2 atmospheres observed in thermogravimetric analysis. A good fitting between the experiments and the calculation results was found.