Molecular beam mass spectrometry with electron impact ionization at 11-70 eV and an electron energy spread of +/-0.25 eV was used to study the structure of a premixed H-2/O-2/Ar (0.26/0.13/0.61) flame without any additives and with 0.14% of diisopropylmethylphosphonate (DIMP), stabilized on a flat-flame burner at 62 mbar. Stable species (H-2, O-2, H2O), as well as atoms and radicals (H, O, OH) were monitored, including phosphorus-containing compounds: DIMP and some intermediates of its destruction, phosphorus oxides and acids. The profiles of the mole fractions of most species, including those of atoms and free radicals were obtained. The calibration coefficients for some species were determined experimentally, and estimated for others. Isopropylmethylphosphonate was detected as a main primary phosphorus-containing product of the destruction of DIMP. It has been shown that bimolecular reactions with hydroxyl radicals and hydrogen atoms, rather than a unimolecular decomposition, provide the crucial initial steps in the destruction of DIMP. A detailed mechanism for the destruction of DIMP in H-2/O-2/Ar flames is suggested.