In dilute acid solution, niobium reacts with Eriochrome Blue Black B (C. I. 201) forming a blue-violet coloured complex. This reaction has been made the basis of a method for determining niobium. Variables for the colour development have been studied. A suitable acidity for the reaction is about 1.5 N in hydrochloric acid. The development of colour takes place slowly at room temperature. Heating in a bath of boiling water hastens the attainment of equilibrium. The coloured species exhibits a maximum absorbance at 560 mμ, but it is more sensitive to measure the absorbance at 590 mμ. In the presence of acetone the colour intensity remains constant for at least 3 hours. The absorbance is directly proportional to the niobium pentoxide concentration in the range 0 to 2.2 p.p.m. The sensitivity, according to Sandell's definition, is about 0.02 μg of niobium per square centimetre at 590 m,u. Nitrate ions decolourize the reagent. The effects of other ions are shown in Table 1 (in the Chinese text),from which it can be seen that iron and molybdenum interfere seriously.