Molecular beam mass-spectrometry was used to study the structure of a premixed H-2/O-2/Ar (0.26/0.13/0.61) flame with and without additives (0.1-1.1% DMMP) stabilized on a flat-flame burner at 43-80 Torr (burner temperature 95 degrees C). The behavior of DMMP in the flame has been studied. Mass spectra of samples taken from flames, and intensity profiles of peaks 18 (H-2 O), 32 (O-2), 40(Ar), 47(PO), 63(PO2), 64 (HOPO), 80, 94, 110, 124(DMMP) amu have been measured as a function of the distance from the burner surface to the sampling probe using a quadrupole mass-spectrometer and electron impact ionization at 12.1-21.6 eV with a spread of electron energy +/-0.25 eV. Intensity profiles of masses 110, 94, 80 pass through a maximum. This shows that the species responsible for these masses are intermediates. PO(CH3)OCH3(OH), P-2(OCH3) are possible intermediates. The profile of the temperature in the flame has been determined by using a Pt-PtRh(10%) thermocouple covered by Ceramobond 569. The effect of promotion on the H-2/O-2/Ar flame by the additive DMMP has been observed. The possible detailed chemical mechanism of the destruction of DMMP is presented.