The photolysis of biacetyl has been reinvestigated. The results are, in general, in excellent agreement with those of Blacet and Bell. Curvature occurs at low temperatures in the Arrhenius plot of [Formula: see text][Biacetyl], and this is attributed to wall reactions, and to the disproportionation reaction[Formula: see text]Azomethane–biacetyl mixtures have been photolyzed to give further information on these points. An activation energy of 8.5 kcal. has been found for the reaction of methyl radicals with biacetyl.