Gas crossover of the product gases, hydrogen and oxygen, through the thin membranes of polymer electrolyte membrane (PEM) water electrolyzers is a major challenge for its further commercialization. It causes safety issues, efficiency losses and triggers degradation mechanisms. In particular, the e ects on gas crossover during PEM water electrolysis are not fully understood yet. In context of the present work, these effects will be investigated experimentally and model-based.In the first part of the dissertation the infuences of different operating conditions:pressure, temperature as well as current density and changes of the electrode structure on gas crossover are experimentally investigated. It is shown that both hydrogen and oxygen crossover increase strongly with current density. However, an increase of the cathode pressure shows no significant infuence on the qualitative extend of this correlation. Thus it is assumed that the underlying mechanisms for this crossover increase are also independent of pressure. This finding stands in contrast to the common explanation in the literature. It is commonly assumed that the crossover increases due to local pressure enhancements. However, since gas transport in general is strongly dependent on pressure this approach contradicts the experimental findings. An alternative explanatory approach is discussed within this work, in which the focus is on the transport of dissolved gases from the catalyst particles through the ionomer to the pore space. Transport limitations on this path, which are independent of pressure, leads to supersaturated dissolved gas concentrations. These concentrations increase with current density, which leads to higher concentration gradients across the membrane and thus to gas crossover increases. The experimental variation of the cathode ionomer content supports this explanation approach. Higher ionomer contents lead to signi cant steeper crossover increases, which can be explained by the increase of the transport resistances due to thicker ionomer films. The investigation of the cell voltage reveals a direct correlation of the increased crossover and mass transport based voltage losses.In the second part, a comprehensive one-dimensional model is formulated to investigate the experimental findings in more detail. The focus is on the previously described theory of supersaturated dissolved gas concentration within the catalyst layers. The simulation results based on literature parameters strengthen this theory. The local profiles reveal that the supersaturated concentrations occur directly at the membrane/catalyst layer interfaces, where the local gas formation is maximal. Furthermore, the complex interactions between ohmic, kinetic and mass transport losses of the catalyst layers are investigated. Finally, the gas crossover is studied by a system consideration with regard to safety and efficiency.