The first derivative curve is used for tolbutamide determination in unit-dose tablets and in combination products. The absorbance contribution from tablet excipient and coexisting components, thiamine and pyridoxine, is thereby nullified. The interference from tolbutamide during thiamine and pyridoxine determination is eliminated by solvent extraction and pH-induced differential spectrophotometry. Thiamine is measured at the isosbestic point of pyridoxine. The latter is determined by the differential absorbance measurement at two wavelengths with the consequent computation of the delta absorbance value.