The present paper provides a quantitative size exclusion chromatography (SEC) method based on the principle of absolute quantification of SEC to solve the problem.The conventional data treatment procedure for the D.R.I. signal of SEC H(V) is improved first by dividing it with the injected sample weight and leads to a novel defined weight normalized distribution Hw(V) and its integral Iw(V).These two distributions reflect the response constant of the sample in addition to the conventional relative total area normalized distribution F(V).The difference of the response constants of the mixtures provides a method to compute their composition from the Hw(V) or Iw(V) of its single component by method of summation.The calculations are carried out in a Mathcad worksheet with the principle of least sum square deviation.The method was applied to mixtures of a kind of industrial asphalt and paraffinic diluents as an example and successful results are obtained.