Abstract: A process for scavenging hydrogen sulphide from a mixed phase system, in particular a hydrocarbon fluid having a liquid phase and a gaseous phase, is described. The hydrocarbon fluid contains a high level of hydrogen sulphide, which can be above 5000 ppm by weight in the gaseous phase of the fluid prior to treatment. Formaldehyde, preferably water, with is added to the fluid. The process is particularly useful for in-line scavenging of hydrogen sulphide from sour crude oil containing very high hydrogen sulphide levels. The hydrogen sulphide content of the crude oil that is delivered to the platform is reduced to safe and commercially acceptable levels and solid by-product formation is insignificant.