AbstractHydroxyl radicals were prepared from the photolysis of N2O at 213.9 nm in the presence of excess H2. The O(1D) produced in the primary photolytic act reacts with H2to produce OH radicals. If CO is also present, then OH can react either with H2or CO:The competition between reactions (1) and (2) was measured by measuring the CO2yield at various values of the ratio [CO]/[H2] at 217–298°K. At 298°K the ratio of the rate coefficientsk1/k2increased with pressure from a low‐pressure limiting value of 14 to a high‐pressure limiting value of 50. The low‐pressure limiting value agrees well with the low‐pressure values found by others. At lower temperatures our high‐pressure values ofk1/k2were larger than deduced from the accepted low‐pressure Arrhenius expression and could be fitted to the expression\documentclass{article}\pagestyle{empty}\begin{document}$$k_1 ^\infty /k_2 = 0.20{\rm exp (} + 3400/RT{\rm)}$$\end{document}The mechanism which seems to fit the results best iswithk1° =k