A diffusion model of space charges in condensed dielectrics is used to explain the slow polarization effect. By separation of variables the diffusion equation is solved in a series form. When the time t is much smaller than the response time

the solution leads to a decay function exp( -t/τ )

which is in agreement with the empirical formula. But for larger t

the decay function transforms into exponential form. A theoretical decay function is given as exp(- t/τ+at 2/τ 2) for t<2 5τ with a=3 613 52. When t≥2 5τ

the decay function is smaller than 0 67%

it is hardly to detect in measurement. A careful experiment for paraffin in room temperature shows that this is an excellent approximate formula.