Thin AgCl layers photocatalytically oxidize water to O₂ under appropriate conditions. The photoactivity of AgCl extends from the UV into the visible light region in a process known as self-sensitization, which is due to the formation of silver during the photoreaction. This silver can be almost quantitatively reoxidized electrochemically, making it feasible that a thin AgCl layer deposited on a conducting substrate can be used as a photoanode for water splitting if coupled with an appropriate photocathode. The silver chloride/silver cluster phase boundary plays a decisive role in the photocatalytic silver chloride electrode system. We have therefore studied this interphase by means of quantum chemical calculations from which we report first results, specifically for the (Ag)₁₁₅(AgCl)₁₉₂ composite. Clusters of semiconducting materials are interesting considering their application as a photocathode in such a device. In this context, we also report the synthesis and properties of luminescent quantum-sized silver sulfide clusters in the cavities of zeolite A. The color of the silver sulfide zeolite A composites ranges from colorless (low loading) to yellow–green (medium loading) to brown (high loading). A low silver sulfide content is characterized by a blue–green luminescence and distinct absorption bands, while samples with medium or high silver sulfide content show an orange or red colored emission and a continuous absorption.