If a solution of polydisperse A- and B-polymers undergoes phase separation, the molar mass distribution of the A-polymers in the A-rich phase becomes different from that in the B-rich phase. This phenomenon is known as fractionation. Here, we compare experimental results on fractionation with Flory-Huggins theory and computer simulations. We study the degree of fractionation f(m) equivalent to rho(A in B)(m)/rho(A) (in A)(m), in which rho(A in A)(m) is the density of A-polymers with molar mass m in the A-rich phase and rho(A in B)(m) that of A-polymers in the B-rich phase. We find that f(m) obeys the simple scaling f(m) similar to exp(-km), where k is a temperature-dependent constant.