Hydrogen spillover was investigated by using the temperature-programmed desorption (TDP) of hydrogen and hydrogenation reaction of toluene over Rh/Al2O3-gamma, diluted by silica, gamma-alumina, HY zeolite and activated carbon (C-ativ), respectively. The strengths and concentrations of the acidic sites of diluent were measured by temperature-programmed desorption (TPD) of toluene, to clarify the correlation between acidic surface properties of diluents and hydrogen spillover. The amounts of acidic sites were ranged as: HY much greater than C-activ > Al2O3 > SiO2, while the strengths of the acidic sites were ordered as: HY congruent to C-activ > Al2O3 > SiO2. It was found that both amount and stability of spilt-over hydrogen correlate with amount and stability of toluene adsorbed on the diluents surfaces. These correlations are due to the mutual dependence of spilt-over hydrogen and toluene adsorption on diluent acidic properties. On the other hand, the remarkable enhancement of toluene conversion is found over both granular mixtures Rh/Al2O3-gamma + HY and Rh/Al2O3-gamma + C-activ. The turnover frequencies of Rh increase with increasing number of acid sites of the diluents, although the diluent is inactive without any metal. It is proposed that the aromatic molecule, adsorbed on acid sites as carbonium ion could be hydrogenated by the hydrogen spilt-over originating from the metal. The hydrogen spilt-over is thus responsible for the increase of the turnover frequency.