Aluminium-doped ZnO layers have been prepared by reactive d.c. magnetron sputtering from Zn:Al (2 wt.%) targets onto unheated substrates (Si, glass, glassy carbon). In dependence on the O2 partial pressure in the argon sputtering gas there exists a narrow process window around a p(O2)/(P(Ar)+p(O2)) ratio of 5-10% which yields transparent, low-resistance layers. The discharge voltage dependence on the oxygen partial pressure is a sensitive indicator for the oxidation state of the target surface and can be used for the regulation of the deposition process. Lower O2 partial pressures yield metallic-like, opaque, but highly resistant layers. Higher oxygen partial pressures lead to transparent but highly resistant ZnO layers. Layers of lowest resistivity (5 x 10(-4) OMEGA cm) and highest optical transmission (90%) have a stoichiometric ratio Zn:O of 1.0 and exhibit the largest grains (almost-equal-to 40 nm) as has been measured by Rutherford backscattering spectroscopy (RBS) and X-ray diffraction (XRD). By comparing the metallurgical Al content (using RBS) in the films with the carrier concentration (Hall and conductivity measurements) we obtain an overall electrical activation of aluminium in the best case of about 60%. We found an exponential dependence of the specific resistance on the ZnO crystallite size which explains the strong dependence of the sheet resistance on the oxygen partial pressure.