The interaction between a photoacid (8-hydroxy-1,3,6-pyrenetrisulfonate, HPTS) and the surfaces of biomaterials and the diffusion of protons along the biomaterial surfaces were examined by following the excited-state proton transfer (ESPT) from the photoacid, adsorbed on the surfaces, to water molecules next to it. We chose two different types of biomaterial surfaces, hydrophobic insulin amyloid fibrils and hydrophilic cellulose surfaces. With the help of steady-state and time-resolved fluorescence techniques, we found that the rate of ESPT from HPTS on insulin fibrils to adjacent water molecules is about (1)/(10) that in bulk water. However, the proton geminate recombination takes place with an efficiency similar to that in bulk water. ESPT from HPTS in wet cellulose to water depends on the weight percentage of water adsorbed by the cellulose. In a semidry sample (<100% weight percentage of water), the ESPT rate is rather low and thus the quantum efficiency of the ESPT is also low within the excited-state lifetime. When the water content is higher, the ESPT rate is almost that of bulk water. We explain these results by the existence of pools of water in cellulose of high water content, in which the triple-negatively charged HPTS molecules desorb from the cellulose surface to these pools. The use of HPTS has allowed us to examine the biological surface and its interaction with water molecules, while obtaining important information regarding the hydration state of the surface that otherwise could not have been obtained. The model that we propose here for the use of photoacids to follow the hydrated state of a given surface is a promising new method of examining the interaction of water molecules with biological surfaces.