We have studied the effects of heat treatments on three CuxS thin films (1.995≤x≤2). Our results suggest that initial heat treatments cause copper in grain boundaries to diffuse irreversibly into the CuxS crystallites. Subsequent heating in hydrogen causes a reduction in surface oxides while the reverse process occurs in an oxygen atmosphere. At a given elevated temperature, the resistivity ρ and charge density P vary with time according to the expressions P=P0e±(t/τ)1/2 and ρ=ρ0e∓(t/τ)1/2 . On the other hand, the mobility is found to be approximately constant at a given temperature during heat treatment.