An improved and sensitive photometric determination of cobalt based on the formation of a coloured chelate with photochemically generated anti-2-thiophenaldehyde 2-pyridylhydrazone (anti-TAPH) is described. The calibration graph is linear over the range 0.017–0.8 μg/ml. A detection limit of 4 ng/ml and r.s.d. of 0.76% was found. Colour development is instantaneous and remains stable for at least 4 hours. The corrected stoichiometry of the chelate was found to be 3:1 (L:M) when the syn-anti ratio at the photostationary state was considered. The influence of reaction variables and the effect of foreign ions are also discussed.