Rate studies of the addition of electron-deficient acetylene to bicyclo[2.1.0]pentane have provided data which are most consistent with the formation of a diradical intermediate. In particular, rate measurements in benzene, ethyl acetate, and acetonitrile showed a negligible solvent effect, the total change in relative rates being less than a factor of 2. More important, the product ratio was independent of solvent polarity. These results are discussed in relation to the classical cycloaddition and ene-type reactions. © 1968, American Chemical Society. All rights reserved.