Electrooptical birefringence in solutions of polybutylisocyanate (PBIC) in tetrachloromethane was studied by applying rectangular voltage pulses with a strength up to 200 kV/cm. At E>8 kV/cm, the Δn(E²) dependence was found to deviate from the linear law. From a comparison of the calculated and experimental Δn(E²) dependences, the dipole moment μ = 500 D and optical anisotropy γ₁-γ₂ = 3.8×10^-22 cm³ were determined for the PBIC molecules with a molecular weight of M = 4.2×10⁴; the data obtained agree with those obtained in the dielectric and dynamic optical measurements. It was shown that the equilibrium flexibility of polymer molecules could affect the Δn(E²) dependence within the range of its deviation from the Kerr effect.