A flow injection manifold incorporating UV-photocatalytic oxidation for the determination of chemical oxygen demand (COD) in freshwater is reported. The method utilises the UV-photocatalytic oxidation of organic compounds instead of conventional heating (used in the standard method), with acidified potassium permanganate as the oxidant. Sodium oxalate, D-glucose and potassium hydrogen phthalate were used as COD standards. A 100 µL sample solution was injected into a 0.3 mol L⁻¹ H₂SO₄ carrier stream containing 0.1 mol L⁻¹ (NH₄)₂SO₄, merged with a permanganate solution (8 × 10⁻⁴ mol L⁻¹) and passed through a 250 cm FEP (fluoroethylene polymer) photo-reaction coil wound around a 15 W UV lamp. The sample throughput was 30 h⁻¹, with an LOD (blank plus 3σ) of 0.5 mg COD L⁻¹ and a linear range of 0.5–50 mg COD L⁻¹ (D-glucose, r² = 0.9966). The method had good precision with relative standard deviations of 2.7% at 5 mg COD L⁻¹ and 1.2% at 20 mg COD L⁻¹ (n = 12) for glucose. Results for a COD certified reference material (QC Demand Quality Control Standard) were in good agreement with the certified COD value. Recovery from Tamar River water samples for all three COD standards was 83.0–111.0% and the COD values determined were in good agreement with those of a permanganate index reference method.