Gas-phase samples of Cr(CO)(5)(C2H4) are prepared in situ by laser irradiation of quantitative mixtures of Cr(CO)(6), CO, and C2H4 In the presence of CO and C2H4, Cr(CO)(5)(C2H4) decays thermally to re-form Cr(CO)(6) by the mechanism of dissociative substitution. Systematic study of the rate of this reaction as a function of partial pressures of CO and C2H4 yields the elementary high-pressure limiting thermal rate constant for unimolecular dissociation of Cr(CO)(5)(C2H4) (k(1)) and the relative rate constant for recombination of Cr(CO)(5) with CO and C2H4 (k(2)/k(3)) Measurements of k(1) and k(2)/k(3) extended over a range of precisely controlled temperatures determine Arrhenius parameters reflecting energetic and statistical properties of these elementary rate processes.