A continuous and simple UV-photometric flow-through biparameter-sensing device has been developed for the simultaneous determination of paracetamol and caffeine at 275 nm. The sensor is based on temporary sequentiation in the arrival of the analytes to the sensing zone by on-line separation using C18 bonded phase beads (the same as that used in the sensing zone) placed into a minicolumn just before the flow cell. The sample containing these compounds is injected into the carrier solution; paracetamol is determined first because it passes through the minicolumn, while caffeine is strongly retained in it. Then, caffeine is conveniently eluted from the precolumn and develops its transitory signal. Using 200 microl of a sample and deionized water as a carrier, the analytical signal showed a very good linearity in the ranges of 10-160 microg ml(-1) and 3.5-50 microg ml(-1) with detection limits of 0.75 and 0.56 microg ml(-1) for paracetamol and caffeine, respectively. If deionized water with the pH adjusted at 12 was used as a carrier solution, these parameters were 25-400 and 4-55 microg ml(-1) with 2.0 and 0.50 microg ml(-1) as the detection limits, respectively. The biparameter optosensor was satisfactorily applied to the simultaneous determination of these two analytes in pharmaceuticals.