A simple model is proposed for the determination of the absolute amount of an analyte when non-destructive detection systems are used in high-performance liquid chromatography and flow-injection analysis. The method requires only a knowledge of the cell thickness, molar absorptivity of the analyte and flow-rate if absorption is measured. The data obtained with a commercial apparatus are consistent with the model both for a compound with well known spectroscopic characteristics (K 2CrO 4) and for common organic substances such as toluene and p-nitroaniline. A systematic error of ca. 18% is present with the detector used for all analytes. The possible origin of this error is discussed.