A new ESR-electrochemical cell with a bubble working electrode is described. To form the bubbles, the electrolyte is pumped through a mercury electrode. Electrolysis occurs in a very reproducible manner in a closed system which enables long term experiments to be carried out in the absence of light and oxygen. The cell is used in the standard three-electrode configuration so that accurate potentiostatic control is maintained during the electrogeneration of ESR sensitive species in the flow through system. Voltammograms can be recorded in the cell to enable optimization of experimental parameters for electrolysis. For short lived species, the electrode is situated immediately before or inside the cavity. In the case of moderately stable species the electrode may easily be placed outside the cavity. The advantages of this electrode are that (i) mercury is utilized as the electrode material, (ii) the electrode surface is always clean and renewed, (iii) the active electrode surface area is very large and hence the electrode is very efficient in generating redox species, (iv) the highly reproducible electrolysis conditions in the closed system make it possible to use lengthy periods of ensemble averaging to improve the signal-to-noise ratio of weak ESR signals. Data on the reduction of fluorescein and methylviologen at the bubble electrode are reported to demonstrate the use and advantages of the new ESR-electrochemical cell. The time constant of the cell has been measured and is in the range of about 1 to 10 s for flow rates up to 8 ml min-1.