In this work, we report a thorough investigation of the reaction of phenols with aryldiazoacetates. Mechanistic studies using different spectroscopic methods and theoretical calculations suggest a hydrogen bond between phenol and aryldiazoacetates, which can be modulated by the phenol acidity. The pKA of phenol and therefore the hydrogen bond plays an important role in a subsequent photoinduced proton transfer reaction to give the formal O-H functionalization product of phenols.