A continuous and simple UV-photometric flow-through optosensor has been developed for the simultaneous determination of a binary mixture of two species with different electric charges present at very different concentrations - ascorbic acid (or acetylsalicylic acid) and thiamine. The sensing device is based on the selective sorption and determination of a cationic analyte on a cation-exchange gel (Sephadex SP-C25) while the other, anionic, analyte is determined in the solution among the interstices of the cation-exchange gel in the same flow cell. The analytes arrive in sequence at the sensing zone, which detects their intrinsic absorbance at 253 nm, as a result of on-line separation by use of a minicolumn filled with the same cation-exchange gel as in the cell, and placed before the flow cell. Thiamine is retained in the minicolumn whereas ascorbic acid or acetylsalicylic acid pass through it and produce their signal as a result of absorbance in the interstitial solution among the resin beads. Thiamine is then eluted from the precolumn, transported to the flow cell, and temporarily retained in the sensing zone from this eluted solution. Calibration graphs were linear over the range 3-50, 25-400, and 300-3000 microg mL(-1) (600 microL sample volume) and the relative standard deviations were 2.56, 1.85, and 1.25 % for thiamine, ascorbic acid, and acetylsalicylic acid, respectively. The proposed method was satisfactorily applied to the determination of binary mixtures of thiamine with ascorbic acid or acetylsalicylic acid in pharmaceutical preparations and semi-synthetic samples.