Electric-field distributions in coplanar-electrode configurations are measured by the linear electro-optic (EO) effects in poled EO polymers. Thermoplastic poling of an isotropic guest-host polyimide system was employed to investigate the electrostatic factors determining the field distribution inside the EO polymer. Electrostatic analysis of the field distribution shows that the introduction of a new boundary condition formed by the polymer surface is necessary to account for field distributions measured by the linear EO effect.