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Thermal decomposition of dimethyl diazomalonate (1) gives two dimers 9and 10, in varying ratio and yield, depending on the reactant / solvent. It is suggested that the ketene 11, formed by Wolff rearrangement, reacts by two different pathways to produce the dimers; a mechanism which seems more reasonable than several alternatives. Independent preparation of 11in the presence of 1did not produce either dimer but with cyclopentadiene the 4-membered ring adduct 7-methoxy-7-(methoxycarbonyl)bicyclo[3.2.0]hept-2-en-6-one (13) was produced. The structure of 13was obtained by X-ray crystallography. Ab initio calculations (6-31G*) indicated that the observed product 13was more thermodynamically stable than the isomeric adduct 16. © 1997 Elsevier Science Ltd.