High-molecular-weight o-cresol resins with narrow-molecular-weight distribution were prepared using a phase-separation reaction system. The prepared o-cresol resins included a trace of monomer, so that the molecular-weight distribution was narrow. In high molecular weight o-cresol resin, the fractions of ortho ortho', ortho-para', and para-para' methylene linkages were close to 25%, 50%, and 25%, respectively. This indicated that the reactivity of the ortho position was equal to that of the para position. It was found that the o-cresol ring was highly activated in phase separation reaction system using H2SO4/acetic acid.