The structure of the title compd. was detd. by x-ray anal. It was found that the cyclopropenyl and the cyclohexadienyl moieties are perpendicular. The C-C-C angle at the spiro atom is 49.64(7) Deg, one of the smaller known for an sp3 carbon. The structure of the cyclohexadienone moiety was analyzed by comparing the C:O distance to the C-C and C:C distances. Using data from previously detd. structures, it was found that the spiro annelation has no extraordinary effect on the structure of this moiety. Ab initio calcns. at B3LYP/6-31G* and MP2/6-31G* levels and NRT anal. were used to analyze the structure of the title compd. and its relevance as a model for vinylidenephenonium ions. We conclude that the neutral spirotrienone is not a good model for the charged bridged ion.