The structures of trifluoroacrylonitrile, F2C=CF-CN, monoclinic, P2(1)/n (no. 14), a = 8.595(4), b = 8.748(1), c = 5.421(1) Angstrom, beta = 102.83(2)degrees, Z = 4, and its thermally unstable isomer trifluorovinyl isocyanide, F2C=CF-NC, monoclinic, P2(1)/n, a = 8.501(2), b = 8.828(2), c = 5.599(2) Angstrom, beta = 101.11(2)degrees, Z = 4 were determined by X-ray crystal structure analysis at 113 and 128 K, respectively, from single crystals grown by partial melting and gradient cooling in small glass capillaries. Selected experimental bond lengths of F2C=CF-CN/F2C=CF-NC are as follows: C=C 1.326(1)/1.304(2), C=N 1.158(1)/1.167(2) Angstrom. The C-F bond lengths of the CF2 group are significantly shorter than those of the CF(NC) and CF(CN) units, respectively. The vibrational frequencies and molecular geometries of this cyanide/isocyanide pair were also calculated by ab initio methods fur comparison with the experimental results, which were found to be in general agreement.