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 consists of: at least one dialdehyde selected from the group consisting of glyoxal, malondialdehyde, succindialdehyde, glutaraldehyde and combinations thereof; at least one base selected from the group consisting of NaOH, KOH, Ba(OH) 2 , CsOH, Sr(OH) 2 , Ca(OH) 2 , LiOH, and combinations thereof; at least one non-nitrogen-containing surfactant selected from the group consisting of alkoxylated alkyl alcohols and salts thereof and alkoxylated alkyl phenols and salts thereof, alkyl and aryl sulfonates, sulfates, phosphates, carboxylates, polyoxyalkyl glycols, fatty alcohols, polyoxyethylene glycol sorbitan alkyl esters, sorbitan alkyl esters, polysorbates, glucosides and combinations thereof; optionally a buffer selected from the group consisting of sodium bicarbonate, potassium bicarbonate, calcium bicarbonate, disodium phosphate, sodium acetate and combinations thereof; and optionally a solvent selected from water, methanol, ethanol, ethylene glycol and butyl carbitol; where synergistically scavenging is defined as the amount of hydrogen sulfide and/or mercaptans scavenged is greater as compared with a composition having a greater amount of dialdehyde where the base and non-nitrogen containing surfactant are absent; wherein: the composition is present in the fluid in a concentration of from 10 to 10,000 ppm; the composition has a pH in the range from 6 to 8.5; and the weight ratio of base to dialdehyde ranges from 25/75 to 0.1/99.9, where the dialdehyde is in a 40 wt% aqueous solution. at least one dialdehyde selected from the group consisting of glyoxal, malondialdehyde, succindialdehyde, glutaraldehyde and combinations thereof; at least one base selected from the group consisting of NaOH, KOH, Ba(OH) 2 , CsOH, Sr(OH) 2 , Ca(OH) 2 , LiOH, and combinations thereof; at least one non-nitrogen-containing surfactant selected from the group consisting of alkoxylated alkyl alcohols and salts thereof and alkoxylated alkyl phenols and salts thereof, alkyl and aryl sulfonates, sulfates, phosphates, carboxylates, polyoxyalkyl glycols, fatty alcohols, polyoxyethylene glycol sorbitan alkyl esters, sorbitan alkyl esters, polysorbates, glucosides and combinations thereof; optionally a buffer selected from the group consisting of sodium bicarbonate, potassium bicarbonate, calcium bicarbonate, disodium phosphate, sodium acetate and combinations thereof; and optionally a solvent selected from water, methanol, ethanol, ethylene glycol and butyl carbitol; where synergistically scavenging is defined as the amount of hydrogen sulfide and/or mercaptans scavenged is greater as compared with a composition having a greater amount of dialdehyde where the base and non-nitrogen containing surfactant are absent; the composition is present in the fluid in a concentration of from 10 to 10,000 ppm; the composition has a pH in the range from 6 to 8.5; and the weight ratio of base to dialdehyde ranges from 25/75 to 0.1/99.9, where the dialdehyde is in a 40 wt% aqueous solution. <EOS>