A process of producing a high molecular weight alkenylphenol polymer which comprises cationically polymerizing an alkenylphenol, such as vinylphenol, in the presence of a nitrile using a cationic polymerization initiator in a homogeneous reaction system. The resulting alkenylphenol polymer is useful as a starting material for preparing a thermosetting resin, a high molecular weight electrolytic substance, an ion exchange membrane, adhesives, glass-fiber-reinforced composite materials, etc.