The Hall mobility &mgr; and conduction electron concentrationnof semiconducting, spray‐deposited CdS thin films are both reduced by chemisorbed oxygen, the former by a larger factor than the latter. The temperature dependence ofnis not affected by chemisorption whereas that of &mgr; is changed from practically no dependence on temperature to a thermally activated dependence. This change is attributed to additional scattering introduced by the chemisorbed ions.