Abstract: The color reaction of cobalt with phenylfluorone (PF) in microemulsion has been studied. Cobalt (Ⅱ) reacts with PF to form a stable 1:2 complex at pH=10.0. Its apparent molar absorptivity is 1.21×10⁵ L/(mol·cm) at 637 nm. Beer's law is obeyed in the range of 0～1.0 μg/mL for cobalt (Ⅱ). The coexistent metal ions can be separated and cobalt can be enriched by sulphydryl dextrane gel (SDG), greatly improving the selectivity and sensibility of system. The method is used to determine cobalt(Ⅱ) in vitamin B₁₂ samples with satisfactory results.