Abstract: Process for purifying gaseous or liquid effluents containing sulphur-containing derivatives (H.sub.2 S, alkyl mercaptans, SO.sub.2).This process comprises: (i) basification of the effluent to be treated at a pH >9 in the presence of a base of general formula M--OH, in which M represents an alkali metal, an alkaline-earth metal, an agent for condensation between the organic part of the compound of formula I below and the sulphur-containing derivative or a phase transfer agent and especially a quaternary ammonium or a ligand; (ii) placing of the product obtained in (i) in contact with a compound of general formula I: X--(CH.sub.2).sub.n --CO--R, in which: X represents: a halogen, a sulphonyl halide of formula X'--SO.sub.2 --R', in which X' represents a halogen and R' represents a C.sub.1 -C.sub.20 alkyl group or an optionally substituted aryl group, an OH group; R represents: a C.sub.1 -C.sub.