The intermolecular [3+2+2] cycloaddition reaction of ethyl cyclopropylideneacetate with alkynes proceeded in the presence of a Ni(0)/PPh3 catalyst, and cycloheptadiene derivatives were obtained. However, in the reaction of 1-cyclopropylidene-2-propanone with phenylacetylene, a cycloheptatriene derivative was isolated. It was anticipated that the isomerization of the initially formed cycloheptadiene derivative led to the formation of the cycloheptatriene derivative. In this paper, we report the isomerization of cycloheptadiene derivatives under thermal, acidic and basic conditions. The stability of the products was also studied by theoretical calculations. The effects of substituents and the mechanism of the isomerization were discussed. © 2009 Elsevier Ltd. All rights reserved.