Fourier transform infrared frequencies v(N=C) and intensities A(N=C) of a series of substituted phenyl isocyanides (16 compounds) have been measured. A A-shaped relation have been found by comparison of v(N=C) (unperturbed by Fermi-resonance) with ó substituent constants: both electron-withdrawing an electron-releasing substituents cause decreases in v(N=C); this result cannot be explained simply within the LFER approach. A satisfactory (R=0.98), positive-slope A1/2(N=C) /ó correlation has been found for the substituted phenyl isocyanides, just opposite to the benzonitrile A1/2(C=N) case. The ó+constant of the isocyano group has been estimated at 0.6.