The use of a composition that includes a metal salt and an oil soluble amine formaldehyde reaction product scavenges H2S that is present in aqueous fluids (e.g. produced water liquid streams), natural gas and in oil and mixtures thereof (e.g. mixed production streams that contain all three phases) better than either component when used alone. The resulting scavenger combination significantly increases the reaction rate and the overall scavenging efficiency, i.e. capacity over each component when used alone, in the same total amount. Non-limiting examples of the metal salt include zinc or iron carboxylates, and a non-limiting example of an oil soluble amine formaldehyde reaction product is the reaction product of dibutylamine with formaldehyde.