Abstract A flow-injection system for the simultaneous determinationof cyanide and thiocyanate is described. A microporous tubular PTFE membrane module with an outer casing was constructed and included inthe system. Cyanide and thiocyanate diffuse thourgh the membrane wall from the phosphoric acid donor stream to a phosphate or carbonate buffer acceptor stream. Percentage transference of cyanide and thiocyanate were 68% and 59%, respectively, at pH 6.0. At pH 8.1, the percentage transference of cyanide was only 19%. The transferred cyanide and thiocyanate are determined by a pyridine/barbituric acid method. Thiocyanate reacts slowly with chloramine-T at pH 8.1, so that cyanide can be determined without interference from thiocyanate. Total cyanide and thiocyanate are determined at pH 6.0. The detection limits ( S/N = 3) are 0.3 μM cyanide and 0.2 μM thiocyanate at pH 6.0, and 5 μM cyanide at pH 8.1. A mechanism for the transference thourgh the membrane is discussed. Bromine interferes with the determination of cyanide and thiocyanate at both pH 6.0 and 8.1. Hexacyanoferrate(II) and hexacyanoferrate(III) interfere at pH 8.1, but not at pH 6.0. Cyanate, oxaloacetate, oxalate, tartrate, albumin, globulin and lysozyme do not interfere.