The oxidation of propylene preadsorbed on the Pt(111) surface has been characterized in oxygen pressures up to 0.02 Torr using fluorescence yield near-edge spectroscopy (FYNES) and temperature-programmed fluorescence yield near-edge spectroscopy (TP-FYNES) above the carbon K edge. During oxidation of adsorbed propylene, a stable intermediate was observed and characterized using these soft X-ray methods. A general in situ method for determining the stoichiometry of carbon-containing reaction intermediate species has been developed and demonstrated for the first time. Total carbon concentration measured during temperature-programmed reaction studies clearly indicates a reaction intermediate is formed in the 300 K temperature range with a surface concentration of 0.55 x 10(15) carbon atoms/cm(2). By comparing the intensity of the C-H o* resonance at the magic angle with the intensity in the carbon continuum, the stoichiometry of this intermediate can be determined unambiguously. Based on calibration with molecular propylene (C3H6) and propylidyne (C3H5), the intermediate has a C3H5 stoichiometry for oxygen pressures up to 0.02 Torr. A set of normal and glancing angle FYNES spectra above the carbon K edge was used to characterize the bonding and structure of this intermediate. Spectra of known coverages of adsorbed propylene and propylidyne served as standards. The spectra of di-sigma propylene, propylidyne. and the intermediate were curve fit as a group with consistent energies and widths of all primary features. Based on this procedure, the intermediate is 1,1,2-tri-sigma 1-methylvinyl. The stoichiometry and temperature stability range of the 1-methylvinyl intermediate formed in oxygen pressures up to 0.02 Torr is identical with the stoichiometry and stability of the same intermediate formed during oxidation of preadsorbed propylene by excess coadsorbed atomic oxygen.