Mass transfer phenomena are known to play a crucial rule in porous heterogeneous catalysts When intraparticle diffusion is too slow to provide a sufficient flux of reactants to the inner parts of the catalyst particles substrate depletion occurs and only the outer parts of the particles are efficiently used for catalysis Quantifying the degree of intraparticle diffusion limitation by means of the Thiele modulus and the related effectiveness factor remains a big challenge Only few techniques combine the necessary spatial resolution and sensitivity to measure in situ the concentration profiles of organic products and/or reagents inside the catalytic particles In this contribution a brief overview is given of in situ techniques that allow such space-resolved activity profiling at the single particle level and their strengths and weaknesses are highlighted The superior sensitivity of fluorescence microscopy is exploited to study the effects of mass transfer limitations on the overall catalytic efficiency in a spatially resolved fashion The epoxidation of bulky substrates over mesoporous Ti-MCM-41 was chosen as a model case It is shown how individual reaction events inside the Ti-MCM-41 particles are visualized and localized and how the obtained reactivity maps yield direct information on the Thiele modulus and effectiveness factor of this specific intraparticle diffusion limited catalytic process Moreover a value for the intraparticle diffusion constant and the intrinsic rate constant of the catalytic reaction could be estimated based on the data obtained by one in situ experiment The presented method is versatile and applicable to a variety of catalytic systems provided that a suitable fluorogenic probe molecule is used which preferably is detectable at the single molecule level after catalytic conversion (C) 2010 Elsevier B V All rights reserved