A method for scavenging hydrogen sulfide and/or mercaptans from a fluid selected from the group consisting of an aqueous phase, a gaseous phase, a hydrocarbon phase and mixtures thereof, the method comprising contacting the fluid with a composition in an effective amount for synergistically scavenging hydrogen sulfide and/or mercaptans, where the composition comprises:
- at least one transition metal salt, and
- at least one water-soluble aldehyde or water-soluble aldehyde precursor;
- where synergistically scavenging is defined as the amount of hydrogen sulfide and/or mercaptans scavenged is greater as compared with a composition where either the transition metal salt or the at least one water-soluble aldehyde or water-soluble aldehyde precursor is absent, used in the same total amount.