Abstract: The response dependence of the flow injection system on the flow rate was minimized by use of a single-bead string reactor. The reactor tube consisted of PTFE tubing (1 m x 0.7 mm) packed with glass beads of diameter slightly smaller than that of the tubing. The system was equipped with an electrochemical detector incorporating two circular vitreous-carbon working electrodes and a Ag - AgCl reference electrode; voltammograms that were independent of flow rate were obtained by means of square-wave voltammetry. The performance of the system was assessed by analyzing samples of potassium ferrocyanide, caffeic acid and vanillic acid. Detection limits were at the nmol level; calibration graphs were rectilinear for, e.g., 100 µM vanillic acid.