The time dependence of the surface potential of an insulating materialis theoretically investigated in the case of simultaneous surface and volume ohmicconduction. When the material presents a constant surface conductivity, the first partof the decrease with time of the surface potential is strongly influenced by theinitial distribution of charges and cannot be used to determine the surfaceconductivity. On the other hand, at rather long times after the surface has beencharged, the potential of the surface follows an exponential variation with time, the expression of the corresponding time constant is given in different cases. This modelis found in good agreement with experiments performed with photo-oxidized epoxy resinsplaced in a humid atmosphere.