Hydrogen (deuterium) adsorption onto surfaces of thin titanium hydride (deuteride) films, TiHx (TiDy), obtained "in situ" in an ultrahigh vacuum apparatus was studied by simultaneous measurement of surface potential (SP) and pressure. It has been found that hydrogen (deuterium) adsorbs dissociatively, forming a weakly bound deposit. At 78 K, reversibly adsorbed, positively polarized adspecies arise. At 298 K, these adspecies incorporate below the surface image plane, thereby inducing a negative charge on the outer surface. Examination of SP isotherms and isothermal adsorption-desorption rates allows determination of the electrostatic parameters of the adsorbates, as well as the activation energies for the studied adsorption and desorption processes. Thermal desorption mass spectrometry allows distinguishing between hydrogen (deuterium) uptake on the surface and in the bulk of titanium hydride (deuteride).