Photoelectrochemical (PEC) biosensors are a rather novel type of biosensors that utilize light to provide information about the composition of an analyte, enabling light-controlled multi-analyte measurements. For enzymatic PEC biosensors, amperometric detection principles are already known in the literature. In contrast, there is only a little information on H + -ion sensitive PEC biosensors. In this work, we demonstrate the detection of H + ions emerged by H + -generating enzymes, exemplarily demonstrated with penicillinase as a model enzyme on a titanium dioxide photoanode. First, we describe the pH sensitivity of the sensor and study possible photoelectrocatalytic reactions with penicillin. Second, we show the enzymatic PEC detection of penicillin.