Reported here are results recently obtained at the Institute of Catalysis with the IRS of adsorbed molecules used to study the mechanisms of catalytic reactions occurring on the surface of heterogeneous catalysts. It has been found that the structure and strength of OH groups complexes with diffusing molecules (M) are the key factors determining the rate of molecules transfer inside the zeolite crystals. Stoichiometry 2M:1OH appears to provide a higher diffusion rate. Original procedures allowing the identification of surface species nature were designed. One of these procedures is based on a quantitative measurement and dosing at a joint adsorption of reagents and probe molecules. Using a joint adsorption of CO and tert-butyl chloride on fluorinated alumina, we have proved that tert-butyl chloride forms complexes with the Lewis sites through the chlorine atoms. The alternating adsorption of aromatic molecules and ammonia proved the aromatic molecule protonation on V2O5/Al2O3 catalyst. A lot of examples of IR mechanistic studies of catalytic reaction on surface are presented. There are butyl alcohol dehydration, methane partial oxidation, n-butyl amine deamination, NOx selective reduction.