A new, completely automated multi-syringe flow injection analysis (MSFIA) system coupled to a gas diffusion unit (GDU) was used for the chemiluminescence (CL)-based determination of sulphide ion in various types of environmental matrices with a high sensitivity and selectivity, and the need for no manual sample pretreatment. Sulphide ions are transferred as H₂S from the donor channel of the GDU to its acceptor channel (AC) through a hydrophobic membrane inserted between the two streams. The solution held in AC replaces the initial sample matrix, which may contain a wide variety of interferents, with one suitable for the CL determination of the analyte. Once sulphide ions have been isolated from the sample matrix, they are determined by their catalytic action on the luminol/H₂O₂ chemiluminescent reaction system. The influence of various chemical and hydrodynamic variables is discussed and the performance of the proposed system compared with that of existing flow systems for the same purpose. Under the operating conditions used, the proposed method features a linear working range of 0.02–2 mg L⁻¹, a limit of detection (3σ_blank) of 0.003 mg L⁻¹, a throughput of 20 samples h⁻¹ and a coefficient of variation of 2.4% (n = 10) for a 1 mg L⁻¹ sulphide concentration. The method was used to determine sulphide in leachates and various types of water samples.