The microstructure of the compound ("white") layer formed on the surface of Fe-4wt.%V alloy, by nitriding in a gas mixture of ammonia and hydrogen at 580 A degrees C, has been investigated by employing light and scanning electron microscopy, X-ray diffraction and electron probe microanalysis. The compound layer is dominantly composed of gamma(|)-Fe4N nitride. Quantitative analysis of the composition data demonstrated that V is present in the compound layer as VN precipitates, i.e. V is not taken up significantly in (Fe, V) nitrides. A mechanism for compound-layer formation has been proposed.