A process and reactor for arsenic fixation in which a first gas stream comprises oxygen and an iron-containing particulate material. The oxygen and particulate material may be fed to reactor through respective first and second inlets. A second gas stream containing one or more volatile arsenic compounds is fed through a third inlet and mixed with the first gas stream and the particulate material to produce a combined gas stream containing the volatile arsenic compounds and the particulate material. The arsenic compounds are reacted with iron in the particulate material as the combined gas stream flows through the reactor to produce solid iron arsenates which are then recovered. The portion of the reactor including the first, second and third inlets is vertically oriented, and the reactor may include a venturi arrangement having a throat at which the second inlet is located.