Preparation of 1,1-difluorobenzocyclopropene (4) and of its 2, 5- and 3, 4-dideuterio derivatives 4a and 4b is reported. Upon ionization in cold fluorosulfonic acid, 4 affords 1-fluorobenzocyclopropenium ion (6). ¹H- and ¹³C-NMR spectra of 4 and 6 are assigned on the basis of the data for the specifically deuteriumlabelled compounds 4a and 6a. Hydrolysis of 6a leads to 2,5-dideuteriobenzoic acid (7a).