Reactions of hydrogenation of the benzene and toluene, in the gas phase, were studied at 60degreesC on Ir/gamma-Al2O3 catalysts of wide dispersion. In the particle size range 3.0-2.0 tim, it was found that the turnover frequency (TOF) values of both reactions were nearly constant with average values of 101.0 x 10(-3) s(-1) for benzene hydrogenation, and 19.3 x 10(-3) s(-1) for toluene hydrogenation. These values were nearly the same as those obtained on Ir/SiO2 catalysts (in the particle size range 7.0-1.0 nm). The agreement of these results obtained on different supports was an important evidence of the structure insensitivity of the two reactions. On the other hand, it was observed that the Ir/gamma-Al2O3 catalysts with Ir particle size below 2.0 nm presented a decreasing in the TOF values of both two reactions (up to one order of magnitude) in comparison with the values obtained on Ir/SiO2 catalysts. This decrease was attributed to the existence of an interaction between Ir atoms and oxygen ions of the alumina that increases the bond strength of chemisorbed unsaturated hydrocarbon, rendering inactive a fraction of surface Ir atoms. In the particle size range 2.0-1.0 nm, only about 50% of the accessible surface Ir atoms were considered as active sites, while for particle size less than or equal to 1.0 nm, about 20-10% of the accessible surface Ir atoms were considered as active sites. It was concluded that there are two types of adsorbed benzene and toluene: a non-reactive form which remains adsorbed on the catalyst without reaction and a reactive one that is hydrogenated.