The diffusion theory shows that the displacement of space charges leads to a slow polarization effect;the response time is proportional to the square of the distance between the electrodes.The theoretical results are confirmed in experiments.A method of differential time domain spectroscopy is used to study the existence and movement of space charges in C6H6 and CH3 (CH2)n CH3.There are one fast component and two slow components in these compounds.Their response times and strength give information about the molecular length and motion at various temperatures.This differential dielectric spectroscopy in the time domain is a very sensitive method for trace analysis of inpurities in highly insulating liquids.