Abstract: A heat pipe (10) comprises an elongated sealed tubular enclosure (20) defining a closed working cavity (50) and having water disposed within the working cavity as the working fluid. The enclosure (20) being formed of a material which reacts with water to generate hydrogen gas. Oxidizer means (30) is disposed within the working cavity (50) in the condenser portion (25) of the heat pipe (10). The oxidizer means (20) comprises a substance which will react with the hydrogen gas generated during operation of the heat pipe to oxidize said hydrogen gas into water. Suitable substances include metal oxides, such as CuO, PhO.sub.2, MnO.sub.2 and HgO, having an electrode potential (or reduction potential) which is higher than that of hydrogen, i.e., the electrode potential (or reduction potential) of such a suitable metal oxide should be a positive potential relative to the zero electrode potential of hydrogen.