A sensitive visual colorimetric method has been developed for the semiquantitative field determination of trace molybdenum. The molybdenum-catalyzed oxidation of ascorbic acid at pH 3.2 in the presence of ophenylenediamine was used as the indicator reaction, which produces yellow quinoxaline derivatives. After a fixed reaction time, the reaction is stopped by readjusting pH to 1 with hydrochloric acid. For the visual determination, the color intensity of the final solution (10 ml) for a water sample is compared to that of a color standard solution containing 0.04 g (4 g l Ψ 1 ) of Mo VI prepared by the same procedure. Two handmade cells of the same size with 10-, 20-, 30-and 40-mm light paths are used in the color comparison for a wide determination range (0.005-0.2 g). The intensity of the color standard is held constant by the adjustment of the reaction time, for example, 10 min at 25 ΨC, with the use of a simple relationship between the reaction time and the field temperature. Molybdenum down to 1 g l Ψ 1 in a 4-ml river-water sample was determined without any special instrument. Analytical performances were evaluated and compared with those obtained by the spectrophotometric measurements. The application to field survey has revealed the distribution of molybdenum concentration along the river streams and a polluted point.