Abstract: In a process for scrubbing contaminant gas and/or solids and/or liquids from a gas stream by diffusiophoretic action hydrogen chloride is used as the diffusiophoretic scrubbing component. The hydrogen chloride may be used per se, may be formed in situ, or may be obtained from fuming hydrochloric acid. Certain contaminants may require the use of formaldehyde to combine with them and produce an easily-removable compound. The scrubbing liquid is usually water, although an alkaline solution may be employed to neutralize the acid. The process has high efficiency with very fine particles (less than 1 micron), odours and sub-micron smokes.