A simple method for the continuous and rapid determination of the chemical oxygen demand (COD) in leaching liquid by flow injection is proposed. A potassium permanganate–sulfuric acid mixture acted both as an oxidant and as spectrophotometric reagent and sodium oxalate was the standard. Samples were injected into a 0.6 mol l^–1 H₂SO₄ stream which merged with the acid–permanganate carrier solution. The redox reaction between permanganate and oxygen-consuming substances was optimally developed at 50 °C on leaching liquids from residues containing heavy metals and at 80 °C on leaching liquids from residues with organic compounds and monitored at 525 nm. A detection limit of 80 µg COD l^–1, quantitation limit of 270 µg COD l^–1, relative standard deviation of 1.3% (calculated at 4.7 mg COD l^–1), a sampling rate of 18 samples h^–1, and excellent tolerance for interferents (chloride ion does not interfere at 8000 mg l^–1, even in the absence of silver salts) are some of the most significant features of the method, which has been exhaustively validated as follows: (a) application to solutions also analysed by an authorised laboratory using the standard method; (b) validation using the certified reference material (CRM) QC PLUS⁺ Demand Quality Control Standard. The comparison of the results found by the proposed method and both those of the interlaboratory exercise and the certified values shows an excellent agreement in all instances.