This article deals with the simultaneous determination of dissolution profiles of two drugs with overlapped spectra, present in the same pharmaceutical formulation. The official procedure for the dissolution profile is adapted to the continuous-flow methodology; the dissolution vessel is connected to an FIA manifold, in which the sample aliquots from the dissolution vessel are treated in order to adjust to the suitable pH and dilution degree to be monitored. The resulting solution is injected into the carrier stream, an acetic acid-acetate buffer at pH 4.3 and forced to the flow-cell of the spectrophotometer. The simultaneous determination of both profiles is based on the first derivative spectra and the zero-crossing mathematical procedure. The empirical profile of the curve is adjusted by regression using different approaches; the three-parameter plot method is selected. The analytical errors, when the concentration of one drug is very low or very high, are also checked. A binary mixture in commercially available formulations of solid oral administration of sulphametoxazole and trimethoprim is presented.