The adsorption of hydrogen and oxygen on Ir/SiO2 and Ir/Al2O3 catalysts of various metallic percentages and different metal particle sizes has been studied by adsorption microcalorimetry. The hydrogen adsorption stoichiometries lay between 1.1 and 2.4, depending on the support and on degree of dispersion. The variation of hydrogen adsorption heat with amount adsorbed revealed significant differences in surface heterogeneity between the various samples. The dependence of hydrogen adsorption stoichiometry and differential heat of adsorption vs. coverage curves on support and on particle size may be related to crystallite size and shape, and to differences in metal-support interaction. On the contrary, oxygen adsorption stoichiometries and differential heat of adsorption vs. coverage curves were very similar for all samples. The amount of oxygen adsorbed at the completion of the monolayer, clearly identified in the differential calorimetric isotherms, coincided with the amount of superficial iridium as determined by a volumetric method.