The cycloaddition of dichloroketene to 3,3-dimethylcyclopentene (5), 3,3-dimethylcyclohexene, 1,5,5-trimethylcyclohexa-1,3-diene, and spiro[2.4]hepta-4,5-diene, and of diphenylketene and tert-butylcyanoketene to 5 has been investigated. The stereochemistry of the product cyclobutanones has been determined by chemical and spectral means, including lanthanide-induced shifts in NMR. It is shown that stereoelectronic effects guide the cycloaddition to cyclohexenes, whereas steric effects predominate in analogous cyclopentene substrates. The steric results are consistent with a nonparallel transition state for addition as required by 2s + 2a or 2s + 2s + 2s mechanisms. Cycloreversion was exhibited by the adduct of 5 with diphenylketene.