High-temperature refined thermomechanical pulp (MDF fiber) was dried, extracted with neutral organic solvent, and reacted with three types of electrophilic reagents (glycidyl methacrylate, maleic anhydride, and succinic anhydride). The adducts formed were further reacted in a hot press with a vinyl monomer in the presence of a free-radical initiator (benzoyl peroxide). Well-conformed boards were produced by the process. Modified boards showed significant improvement in thickness swelling as well as in strength properties compared to control phenol-formaldehyde boards. Glycidyl methacrylate modified/methyl methacrylate boards were the most stable toward the wet- and dry-exposure cycles, whereas succinic anhydride/methyl methacrylate boards showed the lowest water absorption.