In our work, we studied the effect of the triphenyl phosphate flame retardant (TPP) on the flammability of polymethyl methacrylate (PMMA) using probe (microthermocouples) and non-disturbing planar laser-induced fluorescence (PLIF) diagnostic methods. Addition of TPP results in a decrease of the burning rate, reduction of the size of the pyrolysis zone, abatement of the heat flux from the flame to the polymer surface, and also decrease in the concentration of OH radicals in the flame, leading to quenching. Thus, the gas-phase effect of the TPP flame retardant on the flammability of the PMMA polymer was demonstrated. © 2020 Author(s).