Unrestricted density functional theory (B3LYP) calculations have been performed using the 6-311G(d,p) basis sets, to study the hydrogen abstraction from serine by OH in the gas phase. The structures of the different stationary points are discussed. Ring-like structures are found for the transition states. Reaction profiles are modeled including the formation of pre-reactive complexes, and negative net activation energies are obtained, after including basis set superposition error corrections. A complex mechanism involving the formation of a pre-reactive complex is proposed, and the rate coefficients are calculated using conventional transition state theory over the temperature range 250– 350 K. The rate coefficients are proposed for the first time. The following expressions, in L mol 21 s 21 , are found for the alpha channel, beta channel and the overall temperature-dependent rate coefficients: k a ¼ ð2:76 ^ 0:08Þ £ 10 8 exp½ð1389 ^ 8Þ=TŠ; k b ¼ ð1:26 ^ 0:07Þ £ 10 8 exp½ð1524 ^ 17Þ=TŠ and k tot ¼ ð3:91 ^ 0:17Þ £ 10 8 exp½ð1446 ^ 12Þ=TŠ; respectively. q 2003 Elsevier B.V. All rights reserved.