The surface hydrides formed on commercial purity titanium (CP-Ti) and Ti-6Al-4V alloy having been electrolytically hydrogenated in 1 N aqueous sulfuric acid solution were identified to be delta-hydrides. Structure and composition of these delta-hydrides were affected by the thermal oxide films, the additive in the electrolyte, the applied current density and the operating period. The lattice constants of delta-hydrides (a(delta)) observed in this study range from 4.383 to 4.431 Angstrom for CP-Ti and 4.293 to 4.308 Angstrom for Ti-6Al-4V. The nonstoichiometric ratios, x, of delta-hydrides for CP-Ti after hydrogenated are not more than 1.84. X-ray diffractometry (XRD) gave a(delta) and x (indirectly from a(delta)-x curve) to characterize titanium hydrides. Glow discharge optical spectroscopy (GDOS) offered quantitative values such as maximum hydrogen diffusion depth (MHD), average hydrogen concentration (C-A), specific hydrogen uptake (SHU), hydrogen uptake efficiency (HUE), coefficients of inhibiting performance (CIP) and hydrogen absorption rates to elucidate processing parameters on the hydrogenation behavior. (C) 2002 Elsevier Science B.V. All rights reserved.