The photolysis of tetramethylcyclobutan-1,3-dione was investigated in a nitrogen matrix at 4 °K and in solution in cyclohexane at room temperature. Infrared spectra of the reactant after irradiation showed the formation of carbon monoxide, dimethylketene, tetramethylethylene, and a compound with an intense absorption at 1 840 cm-1 which has been identified as tetramethylcyclopropanone. Since the dimethylketene that was formed was also capable of undergoing secondary photolysis, the effect of light (mainly 2 537 Å) on dimethylketene in solution or in a nitrogen matrix, was also studied. It has been found that in solution 0.2 of the molecules of tetramethylcyclobutandione which decompose, do so to give two molecules of dimethylketene, while approximately the same number of molecules decompose to tetramethylcyclo-propanone and carbon monoxide. The remainder of the molecules seem to give tetramethylethylene and carbon monoxide in a concerted process.