A method has been designed for the quantification of 2-oxoglutarate after ion-exclusion high performance liquid chromatography of samples containing high levels of citrate plus isocitrate, which interfere in the quantification of 2-oxoglutarate. The amount of 2-oxoglutarate in a mixed chromatographic peak containing citrate plus isocitrate has been estimated using a diode array detector and calculating the extinction coefficient of these molecules at three wavelengths. The reliability of the method has been ascertained using mixtures of standard organic acids. This procedure has been applied to determine the intracellular content of 2-oxoglutarate in the cyanobacteriumPhormidium laminosum.