The invention relates to the use of thionyl chloride and related materials for dry etching of internal surfaces of metalorganic vapour phase epitaxy (MOVPE) reactors to remove deposits. The method is also useful for the dry etching of process substrates within such reactors for cleaning and processing of those substrates. The invention may be particularly adaptable to chemical vapor deposition reactors used in the manufacture of high brightness LED's based on III-V semiconductors such as GaN and related materials. Features of the process include thermal, UV, and plasma activated dry cleaning, and the use of etchant gases such as COCl2, COBr2, COl2, SOl2, SOCl2, SOBr2, SO2Cl2, SO2Br2, NOCI, NOBr, NOl, S2Cl2, S2Br2, SCI2, SBr2, SOClBr, SOClF and SOFBr, either formed from neat materials or combinations of constituent gases such as CO, SO, SO2 or NO with halogens, to achieve the desired effect.