In the present study, the reaction of NCO(X(2) Pi)) radicals with C2H4 was investigated as a function of temperature (T = 293-741 K) in the pressure range 10-21 Torr. NCO radicals were generated by excimer laser photolysis of ClNCO/Ar mixtures and were detected by laser-induced fluorescence. For temperatures >530 K NCO concentration-time profiles, which were measured for different C2H4 and O-2 concentrations, exhibit a significant reformation of NCO radicals, which is caused by decomposition of the OCNC2H4 adduct in the reaction system. Computer simulations of the system using a kinetic model were applied to analyze the temperature dependence. From these calculations, rate constants for the forward and reverse reaction, NCO + C2H4 reversible arrow/[M] OCNC2H4, hence the equilibrium constant, were determined. The temperature dependence of the equilibrium constant yielded the reaction enthalpy Delta(r)H((526-741K)) = -52.5 +/- 4.5 kJ/mol and the reaction entropy Delta(r)S((526-741K)) = -19.5 +/- 6.0 J/(mol K). In addition, the reaction has been studied as a function of total pressure in the range 2-430 Torr at 294 K. From these measurements the pressure-limiting rate constants k(0) and k(infinity) were extrapolated as well as the broadening factor F-c : k(0) = 1.0(-0.4)(+0.8) x 10(-28) cm(6) s(-1), k(infinity) = 5.5(-0.8)(+0.4) x 10(-12) cm(3) s(-1), F-c = 0.55(-0.15)(+0.11).