Hydrolysis of 3-trimethylsilyl-4,4-dichlorocyclobut-2-ene-1-one (4) in H2SO4 gave small amounts of cyclobutene-1,2-dione (2a) and 3-trimethylsilylcyclobutene-1,2-dione (2d). Photolysis of 2a gave evidence for the formation of the bisketene (CH=C=O2) (1a) in solution for the first time, as identified by the IR band at 2120 cm-1 and the 1H NMR signal at δ 3.20. Thermolysis of 2d gave evidence for the intermediacy of O=C=CHC(SiMe3)=C=O (1d), with an 1H NMR signal at δ 3.43, and the isolation of the unusual ketenyl lactone 6, which could arise by formation of an unsymmetrical [2+2] ketene dimer from 1d, followed by rearrangement and decarbonylation.Key words: bisketenes, cyclobutenediones, ketene dimerization.