Absolute rate coefficients have been determined for the reaction OH+C5H8(1) and two of its isotopomeric variants. Reaction (1) was studied as a function of temperature and pressure in N2, N2/O2 and He buffer gases. At room temperature, a rate coefficient, k1, of (8.56¿0.26)*10-11 cm3 s-1 independent of pressure and buffer gas identity was obtained. An Arrhenius fit to data over the temperature range 250--340 K gave a negative activation energy of -666¿150 cal/mol. HO2 production was inferred from observations of radical regeneration in the presence of NO. The absence of a kinetic isotope effect, together with the specificity of radical regeneration, indicates that reaction proceeds via an addition channel with no significant abstraction component . Our rate coefficient for reaction (1) is 15% lower than the value currently recommenced for atmospheric modeling. ¿ 2000 American Geophysical Union