While many operando techniques focus on gas phase reactions, there is a growing need to look at working catalysts in the liquid phase. Fluorescence microscopy is a promising technique for bridging this phase gap. Like cellular biology, catalytic science may take advantage from the high spatiotemporal resolution and sensitivity of fluorescence microscopy. Earlier applications of fluorescence techniques in the study of diffusion of chemical transformation on inorganic solids are reviewed. The potential of fluorescence microscopy in catalysis is illustrated by data on acid-catalyzed transformation of fluorogenic organics on a mordenite zeolite. (c) 2007 Elsevier B. V. all rights reserve d.