The efficiencies of heterogeneous catalysts could be dramatically improved by rational design. One of the prerequisites for this is a better understanding of the diffusion processes in hierarchical porous structures. Thanks to recent advances in single particle tracking (SPT), it is now possible to follow single molecules with sub-diffraction-limit-precision as they move through catalyst’s porous bodies. However, the pore space of real-life catalysts is highly complex and involves length scales from the nano-level (< 2nm) to fractions of a millimeter, which complicates the studying of diffusion. The aim of this project is to unravel the mysteries of mass transfer inside heterogeneous catalysts, by tracking molecules in well-defined 3D-printed-model-structures and connecting the results with simulations as well as SPT experiments on real catalyst particles.