An effective mobility edge for excitons in two-dimensional systems is shown to exist by comparing photoluminescence ~PL! and PL excitation ~PLE! with excitation-energy-dependent micro-PL spectra. The effective mobility edge of excitons shifts towards higher energies with increasing potential fluctuations. This blue shift is correlated with a sharp decrease of the PLE signal resulting in an extremely large energy separation between the PL and PLE spectra. Numerical simulations using a random potential confirm these results and predict a significant deviation of the PLE from the absorption spectrum. @S0163-1829~97!50636-9#