The dynamics of flourescence quenching of excited state electron donor sensitizers by various pyrimidine and 5,6-dihydropyrimidine substrates was examined. For all of the substrates studied the rate constant of fluorescence quenching (k(q)) increases as the excited state oxidation potential (E-ox*) becomes more negative. The dependence of k(q) on E-ox* in each case is well described by the Rehm-Weller relationship. Fits of the data to this relationship allow for the estimation of the reduction potentials of the substrates (E-red) The pyrimidines 1,3-dimethylthymine, 1,3-dimethyluracil, and 1,3,6-trimethyluracil give E-red values (in CH3CN) ranging from -2.06 (vs SCE) to -2.14 V. Their dihydro derivatives, 1,3-dimethyl-5,6-dihydrothymine, 1,3-dimethyl-5,6-dihydrouracil, and 1,3,6-trimethyl-5,6-dihydrouracil gave E-red values ranging from -1.90 to -2.07 V. The higher E-red values for the dihydropyrimidines compared with their unsaturated derivatives is attributed to aromatic stabilization in the pyrimidines, which is not present in the dihydro derivatives. In addition, the E-red for both the trans-syn and cis-syn diastereomers of the dimethylthymine cyclobutane dimer was examined using the same method. The trans-syn dimer gives an E-red Of -1.73 V and the cis-syn dimer gives an E-red Of -2.20 V. This remarkable difference is attributed to a stereoelectronic effect. The cis-syn dimer anion radical suffers from an unfavorable charge-dipole interaction between the added electron and the O-4 carbonyl group in the remaining pyrimidine ring. In contrast, the trans-syn dimer anion radical shows mainly a stabilizing inductive electron-withdrawing effect of the remaining O-4 carbonyl group. Solvent effects on E-red were also examined. It is shown that the protic solvent, CH3OH, significantly stabilizes the anion radicals, raising E-red by ca. 400 mV over the value in CH3CN.