Molecular-beam mass spectrometry (MBMS) has been used to map the structure of a fuel-rich C2H4/O-2 flame and infer new C2H4 and C2H3 kinetics. Axial profiles of concentration, area expansion ratio, and temperature were measured for a C2H4/O-2/50% Ar (phi = 1.90) flame at 2.667 +/- 0.001 kPa (20 Torr) and 62.5 cm/s burner velocity (300 K). Full concentration profiles were mapped for 42 radical and stable species. Elemental flux balances were within 12%, supporting the data＇s accuracy and validity. Species flux-balance calculations were used to obtain net rates of reaction for the species. Rate constants were determined for H-abstraction from C2H4 by H at 1850-2150 K, and their agreement with theory and previous lower-temperature data leads to recommendation of the nb initio/BAC-MP4 result: C2H4 + H = C2H3 + H-2 k = 4.49 x 10(7) x T-2.12 exp (-13,366/RT) in cm, mol, s, cal, K units. Data for abstraction by OH, combined with literature data, give: C2H4 + OH = C2H3 + H2O k = (5.53 +/- 0.14) x 10(5) x T(2.310+/-0.004) exp [-(2900 +/- 60)/RT] for temperatures between 1400 and 1800 K. Rate constants for vinyl decomposition reaction C2H3 = C2H2 + H were analyzed, supporting the recent recommendations of Knyazev and Slagle [41].