The resistance of a ZnO thin film deposited by rf sputtering exhibits a change of 2 1/2−5 orders of magnitude as the ambient gas is cycled from nitrogen to oxygen. Hall measurements imply that the sputtered films aren−type semiconductor and that the resistance increase caused by oxygen adsorption is related to the decrease of carrier density of the sample. Sample resistivity is found to change with thickness; in oxygen, it decreases as thickness increases, but in a vacuum, it increases as thickness increases. The response time for resistance changes vs environmental changes depends highly on the sample temperature; it is slow at room temperature, but a rapid response can be seen at temperatures higher then 365°C.