The compositions of commercial rosaniline hydrochloride (magenta) and pararosaniline hydrochloride samples have been re-investigated to improve the colorimetric determination of sulphur dioxide. The dyes, purified in small amounts by paper chromatography, each give a linear calibration up to 80 µg of sulphur dioxide, with the hydrochloric acid-formaldehyde reaction at pH 1·1. Of the two dyes, pararosaniline hydrochloride is preferred because of its smaller reagent blank value. Impurities can be readily removed from pararosaniline base by recrystallisation, and this reagent is, therefore, recommended for routine work.