Abstract: Improved catalyzed substrates for use in developing characteristic colors or fluorescence in the presence of peroxidase enzymes (e.g., horseradish peroxidase) are disclosed which include as a rate accelerator a substituted phenol such as a p-halogenated phenol. The complete system typically includes a peroxide type oxidizing agent (e.g., hydrogen peroxide), a chromogenic or flurogenic compound (e.g., ABTS), a buffer and the accelerator compound. Advantageously, the accelerator should provide at least about 50 percent rate enhancement for the substrate, as compared with an otherwise identical, accelerator-free substrate reacted under the same conditions; however, the most preferred accelerator, p-iodophenol, gives enhancements on the order to 1,000 percent. The invention is particularly useful in so-called ELISA determinations which involve an enzyme-linked moiety, and permit detection at very low concentration levels unobtainable with conventional colorimetric substrates.