The polarization response of graphene oxide (GO)-coated planarized optical waveguides is used to determine the complex refractive index of GO film. GO films with thicknesses between 0.10 and 0.71 µm were coated on planarized optical waveguides. GO-coated waveguides exhibit large polarization dependent losses-and the polarization response depends strongly on the GO coating thickness. The response was used, together with finite element analysis, to determine the complex refractive index of the GO film. The complex refractive indices of GO films for both TE- and TM-polarized light at a wavelength of 1550 nm were found to be 1.71+0.09i and 1.58+0.05i, respectively. The uncertainties of nGO and kGO for TE-polarized light are ±0.02 and ±0.03, respectively, whereas the uncertainties of nGO and kGO for TM-polarized light are ±0.05 and ±0.02, respectively.