The scale-up of photoreactions posses challenges due to the non-linear coupling of the radiation field with reaction kinetics and mass transport. A knowledge-based scale-up requires a sufficiently detailed theoretical description of these processes. In this work, a transient, two-dimensional photoreactor model is proposed and used to systematically investigate mass transport limitations in photoreactors, including the effect of transversal mass transport through static mixers and the self-shading effect of the studied homogenous photoisomerization of a spiropyrane. Simulation results of the proposed photoreactor model indicated that mass transport along the direction of light has a major impact. The transversal dispersion would be increased by a factor of 6 by the installation of static mixers, which would allow for a 1.27 fold increase in conversion in an up-scaled photoreactor. A shrinking of the reaction zone was identified when increasing the light power, eventually limiting the reactor performance. Furthermore, a model-based scale-up study emphasized the importance of mass transport for scaling photoreactors.