The oxidation behavior of twenty organic chemicals from four different classes (sugars, benzene derivatives, alcohols and carboxyl acids) were investigated using the official chemical oxygen demand (COD) methods (i.e., permanganate and dichromate methods) and their oxidation levels compared with the theoretical oxygen demand (ThOD) values. In addition, the correlation between the official COD and ThOD values was also examined using the CODMn, Cr method and their relative merits were considered. With the same set of samples, this study also examined the relationship between ThOD values and the response values of a photocatalytic COD sensor which has been developed to overcome several of the problems associated with the official COD methods. The relationship of the response values determined by the photocatalytic COD sensor to those ThOD values was obtained: r = 0.96 for the 20 organic chemicals from each different chemical class.