Abstract I.r. and Raman spectra have been obtained for 3-chlorocyclopropene and 3-chlorocyclopropene- d 3 under a variety of experimental conditions. For mixtures of the two d 2 isotopic species and of the two d 2 isotopic species, only infrared spectra have been recorded. The pattern of gas-to-condensed-phase frequency shifts is interpreted as evidence of hydrogen bonding for the CH hydrogen atoms. A complete assignment of the vibrational fundamentals for the d 0 and d 3 species is derived from the spectra. For the d 0 species these frequencies (in cm −1) are: (a′) 3171, 3029, 1633, 1266, 1141, 913, 713, 591, 345; (a″) 3140, 1034, 1017, 866, 840, 349. With guidance from normal coordinate calculations 59 of the 60 fundamentals for the d 1 and d 2 species are assigned. The full set of 89 observed frequencies has been used to fit an 18-parameter valence-type potential function. In comparison with the force constants for CC and CC stretching of cyclopropene and 3,3-difluorocyclopropene, the CC force constants of 3-chlorocyclopropene show changes similar to those expected for a substitution of a single fluorine atom. This finding is consistent with a σ-acceptor role for the mechanism of the interaction of fluorine and chlorine atoms with the cyclopropene ring system.