The effects of electric fields on excitons in GaAs AlxGa1-xAs quantum wells have been studied using the technique of photocurrent spectroscopy. The quantum wells were imbedded in the depletion region of p-i-n and Schottky barrier photodiodes. A sequence of distinct exciton absorption peaks are seen in the photocurrent spectra for the diodes. Biasing the diodes allows the electric field in the quantum well to be varied. Stark shifts of the excitons were observed in the applied field. For a given field and well width the lowest energy heavy hole (h1) and light hole (l1) excitons exhibited the largest shifts. Some of the higher energy excitons exhibited extremely small shifts. Unallowed exciton peaks became visible in the photocurrent spectra as the electric field was increased. One of these peaks was only visible for wells with widths greater than approximately 120 Å and was shown to arise from mixing between the heavy and light hole valence band subbands. Negative differential resistance regions were observed in the optically excited current-voltage curves for the photodiodes. These were a result of the Stark shifts of h1 and 11. © 1987.