Bisulfite forms adducts with aldehyde and with certain cyclic ketones to give α-hydroxysulfonic acids. This reaction is useful for the purification of aldehydes. The bisulfite adducts precipitate as solids from solution. The reaction can be reversed in base. Examples of such procedures are described for benzaldehyde, 2-tetralone, citral, the ethyl ester of pyruvic acid and glyoxal. In the ring-expansion reaction of cyclohexanone with diazald, the bisulfite reaction is reported to allow differentiation between the primary reaction product cycloheptanone and the main contaminant cyclooctanone.