Density functional theory (DFT) and two-configuration self-consistent-field (TCSCF) calculations including CI were carried out to investigate the pathways of the unimolecular isomerizations of cyclopropanecarbonitrile. Vibrational frequencies calculated at DFT and TCSCF levels of theory were used to estimate frequency factors by transition state theory. Transition states corresponding to both closed shell, concerted and biradical paths in the formation of the main products (cis- and trans-crotonitrile and vinylacetonitrile) were localized. However, owing to Hartree-Fock instabilities in the wave functions of the closed shell (concerted) transition states, it is not clear whether the concerted path is of any physical significance. RRKM calculations were carried out to transfer values of A(infinity) and E-infinity of the isomerizations to A and E-a corresponding to the experimental conditions. Computer modeling, containing the biradical isomerization pathway and the interisomerizations between the products, was carried out, and the results are compared with the experiment. Methacrylonitrile, which is formed in trace quantities, proceeds via a concerted mechanism with a closed shell singlet transition state but with some contribution of a biradical character.