Abstract: Gas purification processes for removing hydrogen sulphide (H.sub.2 S) from a foul gas stream, such as coal carbonization gases, involving a circulating liquid reagent in which the H.sub.2 S is absorbed and converted to sulphur which is recovered and the reagent regenerated and recycled exhibit progressive deterioration of the circulating reagent due to contamination by process by-products. The invention provides a method of decontaminating such a circulating reagent by subjecting a bled-off sidestream to high temperature hydrolysis, so producing gaseous products recyclable to the foul gas stream. Solid products may also be produced which may be recycled to the reagent, perhaps after high-temperature reduction.