Abstract: Online detection of substances with an alcoholic or phenolic hydroxyl group using imidazole and peroxyoxalate chemiluminescence was investigated qualitatively using a flow injection method. The substances tested included six polyphenols, five monophenols and six sugars. After incubation at 80°C with an imidazole buffer (pH 9.5) the substances were detected by peroxyoxalate chemiluminescence. The polyphenols tested (e.g., pyrogallol, purpurogallin, and dopamine) showed the strongest light emission. The sugars with hydroxyl groups (e.g., fructose and lactose) and the monophenols (e.g., phenol, serotonin, and β-estradiol) produced only a weak light emission. Imidazole served two roles, it catalyzed the reaction with the hydroxyl compound and initiated peroxyoxalate chemiluminescence online. A novel reactor formed by packing glass beads into a flow cell (Teflon) of a chemiluminometer improved the sensitivity of light detection.