Very interesting behaviour is shown by chemically deposited CdS thin films for optoelectronic applications after air and vacuum (10−5 Torr) annealing. Vacuum annealing of samples at about 100°C caused the dark conductivity to improve by five orders of magnitude. The dark and photoconductivity of air-annealed (at around 200°C) samples increased by seven and two orders of magnitude respectively. Air-annealed (at 350°C for about 2 min) samples exhibited a very quick photoresponse (<2 s for two decades of photocurrent decay) with σp/σd ≈ 105 for a bias of 10 V, which may be exploited for photodetector applications. Air or vacuum annealing of samples for a minimum of 10–15 min caused the optical transmittance above the band edge to increase by 10%–15% and annealing the sample at 200°C caused the absorption edge to shift towards the longer-wavelength region compared with the as-prepared and 100 and 350°C annealed films.