A photoelectrochemical assay apparatus for determining chemical oxygen demand (COD) of a water sample which consists of a) a measuring cell for holding a sample to be analysed b) a titanium dioxide nanoparticle photoelectric working electrode and a counter electrode disposed in said cell, c) a UV light source adapted to illuminate the photoelectric working electrode d) control means to control the illumination of the working electrode e) potential measuring means to measure the electrical potential at the working and counter electrodes f) analysis means to derive a measure of oxygen demand from the measurements made by the potential measuring means. The method of determining chemical oxygen demand of a water sample, comprises the steps of a) applying a constant potential bias to a photoelectrochemical cell, containing a supporting electrolyte solution; b) illuminating the working electrode with a UV light source and recording the background photocurrent produced at the working electrode from the supporting electrolyte solution; c) adding a water sample, to be analysed, to the photoelectrochemical cell; d) illuminating the working electrode with a UV light source and recording the total photocurrent produced; e) determining the chemical oxygen demand of the water sample according to the type of degradation conditions employed. The determination may be under exhaustive degradation conditions, in which all organics present in the water sample are oxidised or under non-exhaustive degradation conditions, in which the organics present in the water sample are partially oxidised.