A few important examples are described to show that well-chosen molecular probes in organic reactions on heterogeneous catalysts can reveal much about the nature of the active sites and reaction mechanisms. This approach using norbornadiene in hydrogenation casts much light on the reasons why highly dispersed gold is an active selective catalyst. In the new polar mechanism, H-2 undergoes heterolytic fission in the very act of addition to adsorbed alkene, in contrast to the conventional Horiuti-Polanyi mechanism based on prior homolytic dissociation of H-2 on the metal surface.