The existing spectrophotometric method for the determination of total Cu with neocuproine (Nc) does not allow the differentiation of CuI and CuII. It is shown here that the use of a dilute (3.0 x 10(-3) mol dm-3) Nc solution in weakly acidic or neutral media makes the determination of CuI feasible at the 1 x 10(-5) mol dm-3 level in the presence of up to 0.1 mol dm-3 CuII. For the determination of trace amounts of CuI in the presence of 1 mol dm-3 CuII, the latter can be masked by NH3-NH4Cl buffer at pH 10, giving almost the same molar absorptivity for CuI, i. e., 7.5 x 10(3) dm3 mol-1 cm-1. The ability to measure the absorption due to CuI in the presence of an excess of CuII was exploited as the basis of an indirect method for the determination of trace amounts of reductants. Superior to the existing individual methods of determination, which are usually specific for a given reducing agent, the proposed system consists of treating the reductant with the CuII-Nc reagent in ammonium acetate buffered media, followed by measurement of the absorbance of the CuI-Nc chelate at 450 nm. The quantification of a given reductant in the concentration range 1 x 10(-6)-1 x 10(-4) mol dm-3 usually takes 3 min with a mean relative standard deviation of 3%. The molar absorptivity of an n-electron reductant which has reacted stoichiometrically is approximately 7.5n x 10(3) dm3 mol-1 cm-1, i. e., n times that of CuI-Nc. Hence, hydrogen peroxide, ascorbic acid, cysteine, hydroxylamine, hydrazine, thiosulphate, dithionite, mitoxantrone, glutathione, iron(II) and thiourea were determined with theoretical molar absorptivities; carminic acid, sulphite, tin(II) chloride, 2,4-dinitrophenylhydrazine, sodium tetrahydroborate(III) and 2,3-dimercaptopropan-1-ol were determined with the aid of empirical linear absorbance-concentration plots. The proposed spectrophotometric method is rapid and allows the quantification of biologically important reducing agents.