Equilibrium constants for the isotopic exchange reaction between bisulphite ion and the addition product of eight aldehydes and ketones have been measured.At 25 °C., values ranging from 1.021 for acetone to 1.010 for anisaldehyde have been observed. Since constants of this magnitude are to be expected only if the bonding of the sulphur atom is different in the bisulphite ion and the addition product, these results are considered to confirm the carbon-sulphur bond structure of the latter. This conclusion is supported by the small constant, 1.002, obtained for the diethyl sulphate and ethyl hydrogen sulphate isotopic exchange in which the bonding of sulphur is essentially the same in the two reacting species.