Aluminum and magnesium were determined by fluorimetric detection via pre‐column and/or in‐column derivatization with 8‐hydroxyquinoline (oxine) in microcolumn liquid chromatography. Since most oxine complexes are not very stable, the aluminum complex was selectively detected when the eluent contained no oxine, whereas the aluminum and magnesium complexes could be detected when the eluent contained oxine. The sensitivity was improved by on‐column detection, viz., the detection in the presence of the stationary phase. The eluent conditions were optimized in terms of the sensitivity. © 2000 John Wiley & Sons, Inc. J Micro Sep 12: 450–453, 2000