Department of Inorganic and Analytical chemistry, Jadavpur University, Calcutta-700 032 Manuscript received 28 December 1981, revised 11 May 1982, accepted 28 September 1982 Cobalt has been estimated with rubeanic acid in combination with quinoline or collidine at 390 nm and with\(\propto\)-picoline at 410 nm from the yellow colour developed in aqueous alcoholic medium. Beer's law is obeyed within the range of 0.58 ppm to 3 5 ppm of Co,II) with RA and quinoline and 0.5 ppm to 4 ppm of Co(II) with RA and ct.picoline or collidine. Optimum \(p\)H range has been found to be 7 to 8.1. The respective senbiti­vites are 0.0042 µg, Co/cm2, 0 0041 µg Co/cm2 and 0 0034 µg Co/cm2 for RA and quinoline, RA and α-picoline and RA and collidine. Effect of diverse ions, composition of the complexes in solution and optimum working range have been studied.