A rapid and simple preconcentration technique, based on collecting trace elements on a membrane filter and dissolving the membrane filter in an organic solvent, has been applied to the spectrophotometric determination of traces of silicon in water. Silicon, in the range of 1-14 ..mu..g as SiO/sub 2/ in 50 mL of water sample, is converted into ..cap alpha..-silicomolybdic acid and treated with a reductant solution containing tin(IV), L-ascorbic acid, and oxalic acid. The silicomolybdenum blue formed is collected on a nitrocellulose membrane filter in the presence of n-dodecyltrimethylammonium bromide. The membrane and the silicomolybdenum blue are dissolved in 5 mL of dimethylformamide (DMF), and the absorbance due to the silicomolybdate in the DMF is measured at 740 nm against a reagent blank. Phosphate in concentrations similar to that of silicate interferes, but moderate concentrations of arsenate and anionic surfactants and high concentrations of sodium chloride do not interfere. The detection limit is 0.13 ..mu..g of SiO/sub 2/ in 50 mL of sample on a 3..sigma../sub b/ basis.