Thin films of diblock copolymers with cylindrical microdomains oriented normal to the film plane are employed as planar optical waveguides in the Kretschmann configuration (see Figure). Their waveguiding properties were investigated by optical waveguide spectroscopy. The dielectric constants and the film thickness of the block copolymer layer can be independently obtained from fitting between the waveguide patterns for s- and p-polarization and Fresnel calculations.