Photocatalytic water splitting by semiconductor photocatalysts has attracted considerable attention in the past few decades. In this experiment, nanosized titanium dioxide (nano-TiO[subscript 2]) particles are used to photocatalytically split water, which is then monitored by an oxygen sensor. Sacrificial reagents such as organics (EDTA) and metal ions (Fe[superscript 3+]) are also included in the solutions with powder nano-TiO[subscript 2] as photocatalyst to help elucidate the photocatalytic reactions. In solutions containing nano-TiO[subscript 2] and Fe[superscript 3+], the quantity of dissolved oxygen increases, but the quantity of dissolved oxygen decreases upon addition of EDTA. This experiment provides a simple and convenient methodology to indirectly study the photocatalytic water-splitting process and to assess the effectiveness of scavenging toxic chemicals such as organics and metal ions in water. This work should provide valuable experience for undergraduate students in understanding the role of a photocatalyst in water splitting as well as methods for removing toxic substances in water. The proposed photocatalytic system in conjunction with an oxygen sensor and a data logger shows fast response time and good sensitivity for monitoring the photocatalytic reaction. It is more cost-effective and convenient as compared to other conventional methods. (Contains 3 figures.)