A one-dimensional laminar C2H4/O-2/Ar flame was analyzed with a molecular-beam mass spectrometer (MBMS) system. Profiles of concentration, area expansion ratio and temperature were measured for a C2H4O2/56.9% Ar (phi = 0.75) flame at 4.000 +/-0.001 kPa (30.00 Torr) and 30.0 cm/s burner velocity (at 300 K). Full concentration profiles were mapped for 22 stable and radical species, while point measurements were made for seven other species. These mole fraction data are valuable for inference of kinetics and testing of reaction mechanisms. Elemental flux deviations were less than 15% in the flame zone and less than 3% in the post-flame zone supporting the data＇s accuracy and reliability. Flux balance calculations were also used to obtain the net rate of destruction for C2H4 and C2H3. New values of the rate constants for C2H4 + H --> C2H3 + H-2, C2H4 + OH --> C2H3 + H2O and C2H3 --> C2H2 + H were supported by the agreement between measured rates and the rates predicted from measured temperatures, measured mole fractions, and the new rate constants.