As a reactive metal, Ti is prone to surface oxidation spontaneously when exposed to environment containing oxygen-a phenomenon also known as surface passivation. It has also been known that titanium hydride (TiH2) is impervious to oxygen. However, it is not clear to date and there is little published report on how and why hydrogen affects the oxidation of Ti. "Impervious" may be an overstatement because TiH2 does oxidize. Because surface oxygen is also a part of the total oxygen in titanium in addition to bulk oxygen and the passivation film affects the properties of titanium, understanding the surface oxidation behavior of titanium and the effects of hydrogen is thus of considerable interest from both fundamental and practical perspectives. This article studies the effect of hydrogen on the surface passivation of titanium from different aspects including (I) the comparison of oxygen contents in alpha-Ti and TiH2 powders when exposed to air, (II) the oxidation states, their relative fractions, and the thickness of the oxidized layers as a function of hydrogen contents, and (III) the characterization of the passivation layer on the surface by high-resolution transmission electron microscope. The experimental data showed that the presence of hydrogen can indeed make titanium metal less prone to oxidation. The alloying of titanium with hydrogen can result in reduced thickness and the relative fraction of titanium in the form of Ti-IV in the passivated surface, effectively minimizing surface oxidation. The fundamental reason for the effect of hydrogen on the surface oxidation of titanium is discussed and attributed to the difference in oxidation behavior of alpha and delta phases.