A rapid LC method was developed to determine thiourea in natural samples and freshly prepared as well as used tin-baths from the galvanic industry, with LC on a C-18-bonded silica column and with doubly distilled water as eluent. The retention time of thiourea is 1.35 min +/- 5%. UV detection is at 236 nm. Small sample volumes of 10-50 mul allow detection down to 2 mug/l thiourea without any interference by heavy metals or organic compounds and in the case of marine samples by mineral salts. The presented technique should be highly useful for industrial purposes.