Reduction in H₂ of catalyst precursor formed by impregnation of TiO₂, with a rhodium salt produces formation of metal particles on the surface oxide. The use of ¹H NMR spectroscopy has permitted the analysis of hydrogen adsorption on metal particles and hydrogen spill-over from the metal to the sύpport. When the sample is reduced in H₂ at temperatures below 200° C the support does not affect significantly both processes. However, at higher reduction temperatures hydrogen adsorption on the metal and spill-over are hindered. In this temperature range (200-500° C), the formation of oxygen vacancies at the metal-support interface favours hydrogen incorporation into them and formation of Rh-Ti bondings. Both processes induce the electronic perturbation and the loss of hydrogen chemisorption of metal particles (SMSI effect). When samples are reduced above 400° C, blockage of metal adsorption sites by TiO_X suboxide species is also produced. Elimination of electronic perturbation of the metal requires an oxidation of catalysts at 200° C while TiO_X overlayers are removed by oxidation at 400° C. From the NMR analysis, different phenomena participating in inhibition of metal H₂ adsorption have been identified.