A reverse isotope dilution procedure was used to measure the formation of sulphate ion during the air dehydration of sulphited cabbage, carrot and potato at 80°C. The conversion of sulphite to sulphate was found to be of first order with respect to sulphite. The formation of sulphate in the vegetables during dehydration accounts for only part of the sulphate in the dehydrated products; the remainder comes from the scald liquor. The technique for sulphate determination is critically evaluated and the observed yields discussed.