Photolyses of 2,3-bis(trimethylsilyl)-substituted methylenecyclobutenones 22-26 give essentially quantitative conversion to the alkenylketenes 28-32 which have been isolated as long-lived species at room temperature. As predicted by molecular orbital calculations, the methylenecyclobutenones 22 and 26, with carbethoxy and hydrogen substitution on the methylene group, respectively, undergo thermal equilibration with the corresponding alkenylketenes 28 and 32 with equilibrium constants [22]/[28] = 1.0 and [26]/[32] = 0.029, at 100 degrees C. The X-ray structure of the phenyl-substituted alkenylketene 29 confirms the anti-planar conformation as predicted by calculation. The bis(allenylketene) 35 has been made by an analogous procedure. Hydration rates of the allenylketenes show acceleration by the CO(2)Et substituent, but in all cases these are modestly less reactive than the 1,2-bisketene (Me(3)SiC=C=O)(2).