Toluene, phenol, benzyl alcohol, and benzoic acid in alcohols were hydrogenated using Ru/Al2O3 and Pt/Al2O3 catalysts at 120 degrees C under a hydrogen pressure of 6 MPa. The Ru/Al2O3 catalyst exhibited a high activity for hydrogenation of mono-aromatics dissolved in alcohols. The hydrogenation activity of the Pt/Al2O3 catalyst was strongly inhibited by the presence of alcohols. To evaluate the role of solvents, nonpolar and polar solvents, some of which contained oxygen, sulfur or nitrogen, were used for the hydrogenation of benzyl alcohol over the Ru/Al2O3 catalyst. A relationship was found between the hydrogenation reactivity of benzyl alcohol in polar solvents and the 6 value of the solvent, as defined by the difference between donor number and acceptor number. Nonpolar solvents had no effect on the hydrogenation reaction of benzyl alcohol. Polar solvents with negative 6 values, such as methanol, ethanol, and acetic acid, also had no effect. However, polar solvents with positive 6 values, such as acetone, tetrahydrofuran, 1,4-dioxane, and diethyl ether, inhibited the hydrogenation of benzyl alcohol. Since benzene was hydrogenated in preference to benzyl alcohol, the hydrogenation of benzyl alcohol was suppressed when benzene was used as the solvent. No hydrogenation reactivity was found when dimethyl sulfoxide, N,N-dimethyl formamide, and N-methyl-2-pyrrolidone were used as the solvent, since the Ru catalysts were inactivated by the solvents containing sulfur or nitrogen. To enhance the hydrogenation reactivity of aromatic compounds, carboxylic acids were added to the reaction system. The hydrogenation reactivity of benzyl alcohol in the polar solvents was greatly increased by the addition of acetic, butyric, or lauric acid. However, the hydrogenation reactivity was completely retarded by the addition of formic acid with a high relative permittivity. This suggests that the effect of carboxylic acids on improving hydrogenation reactivity can be attributed to the decrease in the relative permittivity of the solvent. On the basis of these results, the roles of solvents and additives in the hydrogenation of mono-aromatic ring compounds can be explained by (1) the delta value of the solvent, (2) competitive hydrogenation between substrate and solvent, (3) the relative permittivity of solvent, and (4) deactivation of the catalyst by solvents that contain sulfur or nitrogen.