hydroxymethylphosphonium salt reaction product is then combined with either an aldehyde or aldehyde donor such as a methylolhydantoin and optionally combined with a quaternary ammonium compound or amine the preferred ph of the composition is adjusted between about 1 and about 9 more preferably between about 2 and about 7 and most preferably between about 3 and about 6 with an appropriate acid or base such as hydrochloric acid or sodium hydroxide if necessary quaternary ammonium compound of the general formula of r 1 r 2 r 3 r 4 n n x n u2212 wherein r 1 r 2 r 3 and r 4 are each independently an alkyl or arylalkyl group having from 1 to 30 carbon atoms and x n u2212 is a mono or polyvalent anion such as a halide a c 2 20 mono or dicarboxylate a borate nitrate bicarbonate carbonate sulfamate a sulfonate sulfate or a phosphate are particularly efficacious examples include didecyldimethylammonium chloride didecyldimethylammonium carbonate didecyldimethylammonium phosphate didecyldimethylammonium sulfamate didecyldimethylammonium citrate c 10 18 alkyl dimethyl benzylammonium chloride or c 10 18 alkyl dimethyl benzylammonium carbonate commercially available products include bardac u2122 2280 carboquat u2122 250 wt barquat u2122 mb 80 and barquat u2122 50 28 all available from lonza inc allendale n j the compositions used in the method of the present invention are particularly suitable for scavenging h 2 s and preventing iron sulfide deposition molar ratios of the composition to the amount of h 2 s present in the system are preferably from 0 25 1 to 100 1 more preferably from 1 1 to 60 1 most preferably from 4 1 to 30 1 of the aldehyde or aldehyde donor preferably from.