Deuterated compounds have received increasing attention in both academia and industrial fields. However, preparations of these compounds are limited for both economic and practical reasons. Herein, convenient generation of deuterium gas (D2) and the preparation of deuterated compounds on a laboratory scale are demonstrated by using a half-sandwich iridium complex with 4,4′-dihydroxy-2,2′-bipyridine. The “umpolung” (i.e., reversal of polarity) of a hydrogen atom of water was achieved in consecutive reactions, that is, a cationic H+/D+ exchange reaction and anionic hydride or deuteride transfer, under mild conditions. Selective D2 evolution (purity up to 89 %) was achieved by using HCO2H as an electron source and D2O as a deuterium source; a rhodium analogue provided HD gas (98 %) under similar conditions. Furthermore, pressurized D2 (98 %) without CO gas was generated by using DCO2D in D2O in a glass autoclave. Transfer deuterogenation of ketones gave α-deuterated alcohols with almost quantitative yields and high deuterium content by using HCO2H in D2O. Mechanistic studies show that the H+/D+ exchange reaction in the iridium hydride complex was much faster than β-elimination and hydride (deuteride) transfer.