An analytical procedure for the rapid determination of the Biochemical Oxygen Demand (BOD) of effluents by potentiometry is presented. The procedure can be completed in a much shorter time, in the order of 30–50 minutes, versus 5 days for the classical procedure. Kinetic equations of BOD measurement and the three most frequently used analytical devices for this analysis are included. The design and performance of the rapid BOD measuring device are shown. The measurement vessel consists of two parts, one is a decantation funnel with connections between its wide and narrow parts, and the other is a piece where both a probe and a gas inlet fit, a design which allows the liquid to flow to the probe without entering into direct contact with the gas. The measurement probe in this rapid analytical procedure is the Oxidation Reduction Potential, ORP, electrode, represented by Pt||KCl(3 M),AgCl(sat.)|Ag.