The exposure of Ta filaments to a pure NH(3) ambient in a hot wire chemical vapour deposition (HWCVD) reactor affects the resistance of the wires. For filament temperatures below 1950 degrees C the resistance increases over time, which is probably caused by in-diffusion of N atoms. Using the filaments in a mixed SiH(4) and NH(3) atmosphere (under SiN(x) deposition conditions) the filaments are hardly affected. Only at the "cold" parts near the electrical contact SiN(x) deposition on the Ta filaments is observed. X-ray diffraction patterns and cross-section microscope images reveal that in a CH(4), H(2) and NH(3) ambient the TaC(0.275)N(0.218) phase is formed on the surface of the filament. Annealing of these filaments at 2000 degrees C causes the TaC(0.275)N(0.218) structure to separate into Ta and Ta(2)C phases. (C) 2009 Elsevier B.V. All rights reserved.