Laser nitriding TiJjAl-IV (Ti64) alloys produces complex microstructures in the solidified melt pool developed during the processing. The microstructures contain mainly titanium nitrides and the a (or u') titanium phase in the matrix. Difficulties have been encountered in identifying the phases and examining the level of residual stress induced by the processing, because of the rapid cooling rate and the nitrogen concentration introduced during the laser gas alloying which have a combined influence in determining the lattice parameters of the phases. The present work has investigated the microstructures formed mainly in a Ti64 alloy under pure argon and various nitrogen concentration environments during the processing, and also to a lesser extent commercial purity titanium (under 20 vol.% N only) studied for comparison purposes, using metallography, X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS). A 5 kW continuous CO2 laser with a spinning beam was used for the processing. As a result of this analysis, the identification of the TiNol phase formed in a Ti64 alloy was confirmed, and the influence of the nitrogen concentration was elucidated. This is considered to be helpful for a better understanding of the laser processing process. Q 1997 Acta Metallurgica Inc.