The invention provides a method for reducing the effects of biological contamination in a hydrocarbon-containing system into which aqueous fluid is injected, the method comprising the steps of: continuously adding a formulation comprising tris(hydroxymethyl) phosphine or a tetrakis (hydroxymethyl) phosphonium salt to the system for a time period of one day or more； monitoring the efficacy of the continuous treatment by an assessment of the extent to which there is any effect on the environment that is attributable to metabolic activity of active bacteria present in the system； and if required, optimising the continuous treatment to ensure that the treatment is having an effect on the metabolic activity of active bacteria present in the system； wherein the tris(hydroxymethyl) phosphine or tetrakis (hydroxymethyl) phosphonium salt is added at a concentration of from 1 to 30ppm, based on the total volume of aqueous fluid added to the system, and wherein the formulation is added to the system at a stage to minimise incompatibility with other any other chemicals that are added to the aqueous fluid. The treatment disrupts the metabolic activity of active bacteria in the hydrocarbon-containing system and can prevent or reduce detrimental effects arising from the presence of active bacteria in the system, such as biological reservoir souring and microbially-induced corrosion.
