The spectral dependence of the refractive index and birefringence of MEH-PPV has been measured. Guided optical waves were excited in the polymer by both grating and prism coupling. By comparing the measured angle dependent reflectivity with that from a numerical model, the optical parameters of the MEH-PPV have been determined. The importance of these parameters for LED design are discussed. (C) 2000 Elsevier Science S.A. All rights reserved.