The optoelectronic properties of oxygen annealed bismuth sulphide thin films have been determined and correlated to the physico-chemical transformations of the powder studied by Differential Scanning Calorimetry (DSC). A maximum in photoconductivity of 0.54 Ω - 1 cm - 1 was Obtained at 170°C with short annealing times. The temperature dependence of the saturation rate indicates the adsorption of oxygen up to 230°C. At higher temperatures oxygen gets chemisorbed and strongly diminishes the overall mobility of the film. The physico-chemical transformations observed by DSC on bismuth sulphide powder show two first order irreversible transitions in the temperature range of 200-300°C. The first has been identified with crystallization of bismuthinite (Bi 2 S 3 ) and BiS 2 . The second transition is quite broad and uncovers the decomposition of BiS 2 into Bi and S and the incipient formation of bismuth oxides and sulphates. At these temperatures, the conversion of bismuth crystals into sulphates and oxides leaves bismuthinite as the only conductive medium, explaining the decrease on conductivity at higher temperatures and duration of annealing. Oxidation of bismuthinite takes place in two stages between 400-500°C, a high activation energy followed by a non-activated and fast oxidation process.