From the photolysis of the title compounds in solution at 185 nm, it is possible to obtain all the possible methylenecyclopropane rearrangement products in about equal amounts. This contrasts with the thermal rearrangement which leads to mostly (>80%) one rearranged product. It is suggested that the photochemical rearrangement proceeds from a π →π*excited state of methylenecyclopropane to the energy surface of a high-lying excited state of trimethylenemethane such as the 1A'1 state (D3h symmetry) but mixes with the lower (1A1) surface and leads to products. The other important photochemical process leads to an olefin and acetylene with retention of the stereochemistry. © 1985, American Chemical Society. All rights reserved.