The structures of the two isomeric y-lactones resulting from acid-catalyzed dehydration of 2- exo-and 2-endo-met hylnorbornene-2-carboxylic acids have been proven by chemical degradation to 2, 2-dimethylnorbornane and 7,;-dimethylnorbornane. The mechanism of formation of the lactones is discussed, and the implications of these results for related systems are presented.