A spectrophotometric study of the reaction of 2-thiobarbituric acid (TBA; 4,6-dihydroxy-2-mercaptopyrimidine) with bismuth(III) is presented. Bismuth(III) forms a coloured complex with an absorbance maximum at 416 nm with TBA in acid solution (pH 1.2–1.7). The maximum absorbance value is achieved within about 25 min. Beer's law is obeyed for a bismuth concentration of up to 10.2 µg ml–1. The molar absorptivity of the complex is 2.4 × 104 l mol–1 at 416 nm and the sensitivity of the reaction is, according to Sandell's calculation, 8.7 × 10–3µg cm–2 per 0.001 absorbance unit. Cobalt(II), nickel(II), magnesium(II), tin(II), lead(II), zinc(II), barium(II), manganese(II) and aluminium(III) do not interfere in the determination. The same procedure can also be used for the spectrophotometric determination of copper(II), which seriously interferes in the bismuth determination. A satisfactory method for the determination of bismuth in the presence of copper by derivative spectrophotometry is, however, reported.