A comprehensive study of thermal decomposition and combustion of a horizontally placed polyoxymethylene (POM) slab was performed. The kinetic parameters of thermal degradation of POM in supposition of two parallel reactions were determined and were used for simulation of the flame spread over the POM. The following main characteristics of the POM slab's combustion were measured: the flame spread rate, the slab's mass loss rate, the width of the pyrolysis zone, the flame height, the temperature profile of the upper and lower surfaces of the slab, the temperature field and the fields of the main flame species concentrations over the burning slab, and the conductive heat flux from the flame onto the fuel surface. It was concluded from analysis of experimental data that two global gas phase reactions may be identified: the reaction of formaldehyde pyrolysis with light combustible gas formed (with the properties close to those of СО) and the subsequent reaction of its oxidation in the flame to the end combustion products (СО2+H2O). This approach was implemented as the coupled combustion model was modified, taking into account a two-step reaction in the gas phase. The results of the calculations made showed good agreement with the experimental data.