To study the mechanism of flame retardancy, counter-flow flames of air and ultrahigh-molecular-weight polyethylene (UHMWPE) with triphenylphosphate (TPP) added and without it were studied at atmospheric pressure. Burning rates were measured. The temperature profiles in the condensed and gas phases were measured by a microthermocouple technique. The burning surface temperature and the temperature gradient in the condensed and gas phases were determined. Dependences of maximum flame temperature on the strain rate for UHMWPE and UHMWPE + 5 wt% of TPP were measured and extinction strain rates were determined. The chemical structure of these counter-flow flames (with and without TPP added) was investigated using the molecular-beam mass spectrometry (MBMS) with soft electron-impact ionization. The stable species H2, H2O, C2H4, CO, O2, CO2 as well as the unstable ones H, OH, HOPO and HOPO2, were identified and their concentration profiles were measured. In adding TPP to UHMWPE, widening of the flame zone, a decrease of the maximum flame temperature, its shifting from the burning surface, reduction of the heat flux from the flame to the polymer surface, reduction of the extinction strain rate, and reduction of H and OH radicals' concentrations were found. In addition, HOPO and HOPO2, the main products of TPP destruction, which catalyze the recombination of H and OH radicals, were found in the flame. Direct experiments conducted demonstrate that the action of a flame retardant in a polymer flame consists in its participation in chain-termination reactions. The study shows that the counterflow flame method can be useful in studying the combustion of polymers containing flame-retardant additives.