Indanedioneketene, a compound resulting from the thermal degradation of the phenyliodonium ylide of lawsone, dimerizes quantitatively to a spiro-oxetanone derivative, a key compound for further transformations. A theoretical electronic structure study of this unusual for alpha-oxoketenes [2 + 2] cyclization reaction both in the gas phase (DFT, MP2) and in dichloromethane solution (DFT), provides support for (a) a single-step, transition-state (involving a four-membered cyclic ring) charge-controlled, concerted mechanism and (b) a [4 + 2] cyclization reaction, not observed but studied theoretically in this study. A parallel study of an open-chain alpha,alpha'-dioxoketene dimerization explains the difference in the stability and reactivity observed experimentally between the cyclic and open-chain products.