A cylindrical reactor with a swirling flow is tested for the production of colloidal sulphur via the oxidation of sulphide ions with a concentration of 2 g dm⁻³ in alkaline solutions at 80 °C. Polysulphides are anodically formed at the outer electrode, coated with lead, whereas hydrogen evolution takes place in the internal electrode as the main cathodic reaction. This geometrical arrangement enables the application of an undivided reactor working with a high current efficiency. Polysulphide's formation occurs over a wide span of potential, from 0 to 1 V against a saturated calomel electrode, at a constant current density that is scarcely dependent on the hydrodynamics. The tests were carried out under potentiostatic control (0.6 V) and colloidal sulphur was obtained by acidification of the final solution. For a 9.6 dm³ min⁻¹ volume flow rate, the space time yield and the specific energy consumption were 29.3 kg m⁻³ h⁻¹ and 27.3 kW h kg⁻¹, respectively, with the current efficiency being close to 100%. Trials under a two-phase flow corroborate the appropriate reactor performance. The colloidal sulphur particles are of high purity, present different shapes and have micrometric dimensions with average sizes ranging from 1.4 to 4.2 μm.