A very simple, highly sensitive and selective spectrophotometric method for the rapid determination of Co(II) in aqeous media containing 30% (v/v) ethanol is. The method is based on the formation of green colour complex by the reaction of Co(II) with 1`-(-2-arsenitro-phenylazo)2`-naphthol in basic media. The effects of amount of reagent, pH, time and temperature have also been studied. Beer`s law is obeyed over the range 0.1 – 40 µg ml-1 of Co(II) with a minimum photometric error of 1.33%. a fair degree of selectivity for Co(II) is achieved by measuring the absorbance of green coloration at 580 nm. The molar absorptivity and sandell sensitivity are also calculated. The proposed method has been applied to determine of vitamine B12 in pure form and in various pharmaceutical formulations. Analusis, 23, 171–174 (1995).