Titanium alloys absorb large amounts of hydrogen atoms (protium) which are desorbed in vacuum at elevated temperatures. The treatment utilizing this phenomenon is called protium treatment. Our studies of (his treatment are reviewed and recent experimental results are introduced. Protium treatment consists of the following three processes for a + beta type titanium alloys: (1) protium absorption, (2) martensitic transformation and hot working, and (3) final treatment for protium desorption and recrystallization, A combination of these processes results in the ultra-fine grains of 0.3-0.5 mum in diameter with high angle boundaries, The fine-grained material shows improved 0.2% proof stress at room temperature and exhibits a superplastic elongation of over 9000%. This is considered to be due to the ultra-fine-grain refinement and the higher proportion of the beta-phase in an a-phase matrix, (C) 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.