The third order nonlinear optical susceptibility, chi ((3)), of an open-shell and alternant polymer based on a pi -conjugated polymer, poly(1,2-phenylenevinylene bearing phenoxyl radicals at 4-position. was evaluated. The real and imaginary parts of chi ((3)) of the poly(1,2-phenylenevinylene) derivative were enhanced three orders of magnitude at ca. 680 nm and reached ca. 10(-9) esu due to the phenoxyl radical formation.