Keywords: Acid Doping Level (ADL), Fuel Cells (FCs), High Temperature (HT), Polymer Electrolyte Membrane (PEM), Phosphoric Acid (PA), Polybenzimidazole (PBI), Raman spectroscopy High temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) are one of the most promising energy conversion devices with zero local CO2 emission. The polymer electrolyte membrane placed between the anode and cathode plays an important role for the overall fuel cell efficiency. The current state-of-the-art membranes used in HT-PEMFCs are PBI-based materials. Compared to Nafion®-based systems used in low temperature PEMFCs, they allow operation at temperatures up to 180-200 °C and do not require water management. Other advantages of the elevated operation temperature are e.g. accelerated reaction kinetics and tolerance improvement against impurities such as carbon monoxide. However, PBI-based membranes are not intrinsically proton conductive and therefore need to be doped with acid in order to ensure proton conductivity. The acid of choice is phosphoric acid due to its non-volatility and thermal stability. Acid doping levels are important since they have a huge influence not only on conductivity, but also onto mechanical and thermal properties of a membrane. Commonly, weighing and titration are used to determine the doping level, but they are bulk sensitive only. In this work, confocal Raman microscopy was used to provide temporal and spatial insights into the acid doping process. It can be performed on the membrane before or after application in a fuel cell due to its non-destructive nature. Our results show that acid doping is a diffusion-limited process. Considerable changes in spatial distribution of dopant were observed within the membrane before reaching saturation. Further monitoring of diffusion of phosphoric acid after removal from the doping solution shows that the diffusion continues even after months. Confocal Raman microscopy offers the option to gather information non-destructively and contact-free from within the membrane. The insights into the phosphoric acid doping process of PBI-based membranes presented here are of great importance for the manufacturing and optimization process of high temperature fuel cells and their overall efficiency.