The fading behaviour of a variety of naphthol azo dyes with an excess of sodium sulphite was investigated spectrophotometrically at 30 degrees C in the pH range from 3 to 10. The fading occurred most efficiently under neutral conditions. The azo dyes employed were grouped distinctly into the following three types on basis of the observed spectroscopic characteristics: addition type (A-type), in which the dyes had relatively faster rates had isosbestic points at near 440 nm; reduction type (R-type), in which the dyes had slower fading rates and no clear isosbestic point; and no reaction type (N-type), in which the dyes showed no fading under the experimental conditions. The results suggested that the fading of the azo dyes with sodium sulphite occurred through a Michael addition of a sulphite ion to a conjugate active enone moiety in the hydrazone tautomer (A-type), or to a conjugate iminone moiety in the same tautomer (R-type).