GaAs/AlGaAs multiple quantum well (MQW) waveguides are investigated using the grating coupling technique. Large polarization dependent effects of the two-dimensional excitons are seen in the TE/TM dispersion relation of single mode slab waveguides. By treating the MQW as a layered composite dielectric, we deduce the oscillator strengths of the heavy and light hole excitons, and the background dielectric constant of the MQW without the excitonic contribution, for fields polarized parallel and perpendicular to the MQW layers.