A method for preparing crystalline titanium(IV) bis(hydrogenphosphate) monohydrate, Ti(HPO4)2-H2O (α-TiP), was established by the raction of titanium(IV) hydroxide, TiO2nH2O, or titanium(IV) oxide, TiO2 (rutile and anatase type), with orthophosphoric acid in an autoclave. In the titanium(IV) hydroxide and titanium(IV) oxide systems, the preparation of α-TiP was strongly dependent on mixing ratio (P2O5/TiO2), heating temperature, and heating time. The recommended method for the prearation of α-TiP is mixing ratio P2O5/TiO2=1.2~1.5, heating temperature 200~225oC, and heating time 5 h. α-TiP prepared by the present method is not soluble in distilled water and hydrochloric acid (0.1 mol-dm–3), but slightly dissolves in sodium hydroxide and ammonia solutions. α-TiP effectively adsorbed ammonia in an aqueous solution and ammonia gas. © 1990, The Chemical Society of Japan. All rights reserved.