Abstract: Details are given (with diagrams) of HPLC and flow injection systems in which the column eluate, or the sample-containing carrier stream emerging from a damper coil, is treated with 0.5 M imidazole buffer of pH 7.5 (adjusted with HNO3) - acetonitrile (1:4) and, after passing through a mixing coil, 1.0 mM bis-(2,4,6-trichlorophenyl) oxalate and 20 mM H2O2 in acetonitrile. After the solution has passed through a further mixing coil, its chemiluminescence is measured with a Soma S-3400 luminescence detector. This system allows mobile phases with a range of pH values (e.g. 2.0 to 8.0) to be used, as demonstrated by the sensitivity (1 fmol) achieved for perylene as test analyte. Improved signal-to-noise ratios compared with an earlier chemiluminescence system were obtained in the HPLC of 1-pmol amounts of nitro- and nitroso-pyrenes on a Nucleosil ODS column (25 cm x 4.6 mm) with 2.0 mM imidazole - HClO4 (pH 4.75) in acetonitrile (1:4) as mobile phase and the cited detection system.