The photocycloadditions of several alkyl ketones to 1,2-dicyanoethylene, maleic anhydride, and 1-methoxy-1-butene are described. The cycloadditions are stereospecific in the case of the dicyanoethylenes but nonstereospecific in the case of 1-methoxy-1-butene. Photosensitized geometric isomerization of the ethylenes is an important side reaction. A discussion is given of the scope of these photocycloadditions and the factors which control yields and stereospecificity. © 1969, American Chemical Society. All rights reserved.