The role of metal oxide semiconductor (MOS) gas sensing mechanisms has been discussed. It is demonstrated that the sensing characteristics of metal oxide semiconductor gas sensors can be improved by controlling fundamental factors which affect its receptor and transducer function. MOS gas sensor properties are deeply related with type of semiconductor, boundary layer thickness and microstructure. The sensitivity can be drastically modified by reorganizing microstructure and its construction elements, as well as by using precious metal or other MOS promoters.