A capillary optode for determination of mercury(II) in aqueous solution has been developed. The sensors are composed of the capillary glass tubes with a chemically sensitive coating on the inner surface, consisting of a 4-(2-thiazolylazo) resorcinol (TAR) and Nafion® composite thin film. When a sample solution is passed through a capillary tube, the mercury(II) ions forms complexes with the immobilised TAR resulting in a change of the colour of the inner surface of the capillary which, in turn, can be monitored spectrophotometrically. The characteristics of these sensors including effect of flow-rate and pH, reproducibility, sensitivity, linear range, life time and selectivity are discussed.