The photolysis of acetone has been investigated at room temperature using low pressures and high intensities. In addition an investigation was made of the photolysis of azomethane–acetone mixtures. The results indicate that the curvature at low temperatures of Arrhenius plots of[Acetone] is due to two causes (a) a reaction between methyl radicals and adsorbed acetone and (b) to the occurrence of the disproportionation reactionConfirmatory evidence for wall effects was obtained from experiments at low pressures and higher temperatures.