Free radical-initiated copolymerization of diphenylvinylphosphine oxide(DPVPO) with acrylonitrile(AN) and vinyl acetate (VAc) were studie& The monomer reactivity ratios for the monomer pairs determined at 70℃ using benzoyl peroxide as an initiator were: r₁(AN)=3.8, r₂(DPVPO)=0.16 and r₁(VAc)=0.38, r_{12/sub>(DPVPO)=1.7. The values of the Alfrey-Price constants, Q and e, for DPVPO were calculated to be 0.068 and 0.49, respectively, from the AN-DPVPO system, and 0.059 and 0.44, respectively, from VAc-DPVPO pair. The reduced viscosity of copolymers decreased as the content of DPVPO units increased for both systems.}