Photolysis of methoxv acetone at 3130 Å. led to acetone and formaldehyde as the important products. A small amount (about 1/20th of the acetone) of carbon monoxide was the only other significant product. Although estimates of the yield of formaldehyde led to lower values than for acetone, it is likely that methoxv acetone decomposes mainly to one molecule each of acetone and formaldehyde. The quantum yield of acetone was 0.32. This value was unaffected by changes in pressure and was only slightly affected by an increase in temperature. Addition of biacetyl or nitric oxide had no observable effect. Oxygen had no effect at 105° but at room temperature a decrease in the yield of acetone was noted. It is believed that acetone (and formaldehyde) are formed by the Norrish type II intramolecular process. The effect of oxygen at room temperature may be due to changes in the surface of the walls of the reaction cell. © 1962, American Chemical Society. All rights reserved.