Detailed numerical simulations of premixed and non-premixed C2H4/air flames were conducted, using six available kinetic mechanisms. The results help assess differences between these mechanisms and are of interest to proposed hydrocarbon-fueled SCRAMJET concepts, in which C2H4 Can be expected to be a major component of the thermally cracked fuel. For premixed flames, laminar flame speeds were calculated and compared with available experimental data. For non-premixed flames, ignition/extinction Z-curves were calculated for conditions of relevance to proposed SCRAMJET concepts. Results revealed a large variance in predictions of the kinetic mechanisms examined. Differences in laminar flame speeds as high as factors of 2.5 were found. For the conditions investigated, computed ignition and extinction strain rates for non-premixed flames differed by factors as high as 300 and 3, respectively. This indicates that while there are differences in high-temperature kinetics that control flame propagation and extinction, discrepancies in low-temperature kinetics that control ignition can be even more significant. Sensitivity- and species-consumption analyses indicate uncertainties in fuel kinetics and, most importantly, on the oxidation Of C2H3 and the production of CH2CHO, whose kinetics are not well known and can crucially affect production of the important H radicals. These findings stress the need for experimental data in premixed and non-premixed configurations that can be used to assess these phenomena and provide the basis for a comprehensive validation.