The photoresponse of polycrystalline ZnO films generally contains both a true photoconductivity and a contribution from surface structural changes which can alter the surface conductance via the chemisorption and photodesorption of an active ambient-gas component. The surface structural changes can increase the conductance by up to seven orders of magnitude for a 500 nm thick sample when illuminated with an intensity of 4 x 10(16) photon cm(-2) s(-1) UV light, but the response times are very slow. Newly prepared samples exhibit a fast response characteristic such as a true photoconductivity, but after exposure to air for several months the slower response dominates. Thus if used as a photoconductor, oxygen adsorption or desorption from the surface of a ZnO film is particularly problematic, but some surface stability can be achieved by adding nitrogen to the surface layers of the ZnO film and these samples exhibit more normal and improved photoconducting behaviour.