The time dependence of the surface potential of an insulating material
is theoretically investigated in the case of simultaneous surface and volume ohmic
conduction. When the material presents a constant surface conductivity, the first part
of the decrease with time of the surface potential is strongly influenced by the
initial distribution of charges and cannot be used to determine the surface
conductivity. On the other hand, at rather long times after the surface has been
charged, the potential of the surface follows an exponential variation with time, the
expression of the corresponding time constant is given in different cases. This model
is found in good agreement with experiments performed with photo-oxidized epoxy resins
placed in a humid atmosphere.