The characteristics of a new superconducting wire material, consisting of a 7 μm carbon filament substrate thinly coated with niobium carbonitride, are reported. These filaments are coated (∼ 1000 at a time as a yarn) by a co-CVD process in which the desired carbonitride is deposited as a compound. Several coating compositions from NbC to NbC.75N.15have been studied. At each composition several coating thicknesses from 300 to 3000 Å were prepared. Although our present coating compositions are not near NbC0.3N0.7, which has previously been determined as optimum for this pseudobinary system, we nevertheless have obtained excellent superconducting properties. Zero-field critical current densities of 2 × 106A/cm2in the coating have been obtained, and there is no discernable ac loss at frequencies up to 50 kHz. In addition to the superconducting measurements we have also performed chemical, x-ray diffraction, and electron microscopic analyses on the coated filaments.