The role of H2O vapor on the disproportionation/combination ratio (k(d)/k(c)) Of C2H5 radical reactions in the gas phase has been investigated experimentally and compared to that observed previously in aqueous solution. At added water pressures of 20 kPa (15 Torr), the k(d)/k(c) ratio remains unchanged from that observed in its absence. The relationship of the invariant ratio to bonding and solvation phenomena are discussed.