Herein, we describe a protocol for photocatalytic testing of dispersed photocatalysts in aqueous medium. It covers the preparation of suitable catalyst dispersions, a photoreactor set-up that allows for accurate control of temperature and light intensity as well as multiple methods for (time-resolved) detection of oxygen, hydrogen and hydrogen peroxide (possible water splitting products). It is important to note that the used photoreactor configuration is simple and allows obtaining reproducible results, while requiring only small volumes of photocatalyst dispersions. We hope that his protocol can aid other researchers in conducting reproducible and insightful tests of heterogeneous photocatalysts for water splitting.