The phase separation and fractionation for phenol-formaldehyde resins of the random novolak type (r-NR) were investigated by using r-NR/acetone/hexane and r-NR/acetone/cyclohexane ternary systems. The fractionation efficiency was elucidated by the partition coefficient, sigma, which was derived from the theory of Flory and Huggins. The fractionation based on molecular structure and molecular weight occurred at the same time. The partition coefficient represented the fractionation efficiency based on molecular weight as well as on molecular structure.