Thiamine is presently one of the most attractive substrates used for sensitive fluorometric measurements of peroxides. Thiochrome (TC), a highly fluorescent product, is formed in enzyme-mediated oxidations. It is assumed that H2O2 is nearly quantitatively converted to TC. The reaction cannot differentiate H2O2 from many other peroxides such as methylhydroperoxide (MHP); to perform differential measurements, H2O2 can first be selectively destroyed by a suitable catalyst such as MnO2. In substituting Ag2O for MnO2 to accomplish the selective destruction of H 2O2, we achieved the stated objective but were puzzled by a 3-fold increase in the MHP response in the presence of Ag2O. It was soon discovered that traces of dissolved Ag+ and Hg2+ can dramatically increase the yield of TC in this reaction from either H 2O2 or MHP; the normal yield in fact is only 20%. We present here a reaction scheme and kinetic model that adequately describes this behavior and should provide a path to substantially increase the sensitivity of this important assay method.