A composition for scavenging hydrogen sulphide and mercaptans in hydrocarbon streams, said composition comprising an oxazolidine compound and an additive, the additive comprising: (i) the reaction product of a compound with a compound, wherein: - Compound is a polyisobutenyl succinic anhydride (PIBSA), - Compound is a nitrogen-containing compound formula: H 2 N-[(CHR 3 -(CH 2) p -CHR 4) q -NH] m -H Wherein: R 3 and R 4, which are identical or different, represent hydrogen or an alkyl or alkenyl group comprising from 1 to 4 carbon atoms, q is an integer varying from 1 to 3, m is an integer varying from 1 to 10 and p is an integer equal to 0 or 1, or (ii) the reaction product of a compound with a compound, followed by a quaternization reaction of the reaction product of and with a compound, wherein: - Compound is a a polyisobutenyl succinic anhydride (PIBSA), - Compound is an alkanolamine comprising one hydroxyl function, one primary amine function and one tertiary amine function and from 4 to 16 carbon atoms, - Compound is a quaternizing agent, wherein the composition comprises from 19 to 99%wt of oxazolidine compound(s) and from 1 to 50%wt of said additive(s), based on the total weight of the composition, and wherein the oxazolidine compound is selected from bisoxazolidines replying to formula: wherein - n is an integer ranging from 1 to 6; - R1 and R2, identical or different, are selected from a hydrogen atom and a linear, branched or cyclic alkyl or alkenyl groups having from 1 to 6 carbon atoms. (i) the reaction product of a compound with a compound, wherein: - Compound is a polyisobutenyl succinic anhydride (PIBSA), - Compound is a nitrogen-containing compound formula: H 2 N-[(CHR 3 -(CH 2) p -CHR 4) q -NH] m -H Wherein: R 3 and R 4, which are identical or different, represent hydrogen or an alkyl or alkenyl group comprising from 1 to 4 carbon atoms, q is an integer varying from 1 to 3, m is an integer varying from 1 to 10 and p is an integer equal to 0 or 1, or - Compound is a polyisobutenyl succinic anhydride (PIBSA), - Compound is a nitrogen-containing compound formula: H 2 N-[(CHR 3 -(CH 2) p -CHR 4) q -NH] m -H R 3 and R 4, which are identical or different, represent hydrogen or an alkyl or alkenyl group comprising from 1 to 4 carbon atoms, q is an integer varying from 1 to 3, m is an integer varying from 1 to 10 and p is an integer equal to 0 or 1, or (ii) the reaction product of a compound with a compound, followed by a quaternization reaction of the reaction product of and with a compound, wherein: - Compound is a a polyisobutenyl succinic anhydride (PIBSA), - Compound is an alkanolamine comprising one hydroxyl function, one primary amine function and one tertiary amine function and from 4 to 16 carbon atoms, - Compound is a quaternizing agent, - Compound is a a polyisobutenyl succinic anhydride (PIBSA), - Compound is an alkanolamine comprising one hydroxyl function, one primary amine function and one tertiary amine function and from 4 to 16 carbon atoms, - Compound is a quaternizing agent, - n is an integer ranging from 1 to 6; - R1 and R2, identical or different, are selected from a hydrogen atom and a linear, branched or cyclic alkyl or alkenyl groups having from 1 to 6 carbon atoms.