A flow analysis method based on the on-line concentration and direct absorptiometric measurement of the ion-associate species formed between molybdosilicate and Malachite Green on Sephadex LH-20 gel, is described for the determination of silicic acid in highly purified water. The increase in the attenuance of the coloured species, which was concentrated on-line onto the gel packed in a flow-through cell, could be measured continuously with a spectrophotometer at 627 nm. The coloured species on the gel was easily desorbed with a mixture of sulphuric acid and acetone, and the gel-packed flow-through cell could be used repeatedly. The concentrations of silicic acid in 4–5 samples of highly purified water were determined in less than 1 h. The detection limit was 0.1 µg dm^–3 of Si using water samples with a volume of about 5 cm³. The total amount of silicate-silicon in highly purified water was determined after treating the samples with ammonia solution.