Use of hydrogen as a temporary alloying element in titanium alloys is an attractive approach for enhancing processability, and also for controlling the microstructure and improving final mechanical properties. In this study, the alpha + beta titanium alloy, Ti-6Al-4V, was hydrogenated with hydrogen levels of 0.1, 0.3 and 0 5 wt%. The microstructure, phases and phase transformations were investigated by optical microscopy, X-ray diffraction and transmission electron microscopy The results showed that the hydrogen addition had a noticeable influence on the microstructure of Ti-6Al-4V alloy. Hydrogen stabled the beta-phase and leaded to the formation of hexagonal close packed alpha' martensite as well as face-centered cubic delta hydride. Microstructural evolution and hydride precipitation mechanism in hydrogenated Ti-6Al-4V alloy was revealed. (C) 2009 Professor T Nejat Veziroglu Published by Elsevier Ltd All rights reserved