This critical review describes the reactivity of heterogeneous catalysts from the point of view of four simple, but essential for Chemistry, molecules (namely dihydrogen, carbon monoxide, nitrogen monoxide and ethylene) that are considered as probes or as reactants in combination with "in situ" controlled temperature and pressure Infrared spectroscopy. The fundamental properties of H(2), CO, NO and C(2)H(4) are shortly described in order to justify their different behaviour in respect of isolated sites in different environments, extended surfaces, clusters, crystalline or amorphous materials. The description is given by considering some "key studies" and trying to evidence similarities and differences among surfaces and probes (572 references).