The photolysis of acetone has been re-examined at temperatures between 107 and 225°C, and pressures between 0.13 and 133 mbar. In a Pyrex reaction vessel the wall reaction, CH3·+(CH3COCH3)ads→ CH4+ CH3COCH2·, contributes significantly to methane formation at acetone pressures below 10 mbar. The rate of this reaction is proportional to the methyl radical concentration in the gas phase, and varies markedly with the nature of the surface. On PTFE and PbO surfaces the rate of reaction appears independent of acetone pressure in the range 0.13–2.7 mbar, but on Pyrex it increases slowly with increasing acetone pressure. On Pyrex surfaces the reaction has an activation energy of 6.6 ± 1.0 kcal mol–1 and is always faster on a per collision basis than the analogous reaction in the gas phase.