In most cases, products from titanium alloys are subjected to final treatment in a vacuum or inert gas for removal of residual hydrogen and residual stress in the metal. During treatment in the reduced pressure or inert atmosphere, titanium alloys absorb impurities from the process gas environment. The uncontrolled saturation of subsurface layers of the metal leads to increased brittleness of the surface layers; thus, in practice, it is necessary to remove all or part of the hardened surface layers of the metal. In this regard, it is shown that the normalized hardening of the subsurface layers increases the resistance to fracture under cyclic and static long-term loading of the metal products up to 4 mm thick from α- and pseudo-α- titanium alloys. Hardening is caused by thermochemical surface treatment from a controlled gas environment that contains oxygen. It was experimentally shown that fatigue resistance of α- and pseudo-α- titanium alloys depends on the ratio of hardening and depth of the hardening zone. This dependence showed a peak when at the optimal level of solid solution hardening. Any additional concentration of elements in the layer (increased hardness) will lead to a drastic reduction of fatigue characteristics of titanium alloys. The effect of increasing longevity of α- and pseudo-α- titanium alloys and oxygen solid solution on the “optimal” level of subsurface layers under different loading conditions was confirmed experimentally. The loading of the specimens was performed to evaluate pure bending; rotating bending; cyclic stretch, and long-term static loading at 1000 h. The increase in fatigue strength of the titanium alloys samples, saturated not only with oxygen but also with other interstitial elements (N and C) at a specified ratio of surface hardening to depth of hardened zone, was determined.