Proof-of-concept: A polyoxometalate cluster has been used as a highly effective peroxidase mimetic to catalyze H2O2 oxidation of various peroxidase substrates to give colored solutions, leading to a simple, fast, sensitive, and selective assay for H2O2 with a linear range and detection limit of 1-20 and 0.4 mm. This assay was also used for glucose detection both in buffer solution and diluted serum with a linear range and detection limit of 1-10 and 0.5 mm.