The influence of annealing temperature on the sheet electrical resistance in the dark (R rectangle d) and the photosensitivity (S) of the chemically deposited PbS thin films (thickness, 0.12-0.20 mu m) was studied. By thermal treatment in air at 75-150 degrees C, the parameters were increased, but after keeping the samples on anhydrous CaCl2 or P2O5, they decreased. The process is reversible and can be interpreted as a sorption/ desorption of H2(O) vapours. The concentration of sO(4)(2-) ions in the annealed films as determined by spectrophotometric titration, was 76-200 mu g cm(-2). By X-ray diffraction analysis, an increase of the average crystallite size and a decrease of the microstrains were observed.