Using a novel photocurrent difference spectroscopy, six conduction subbands have become observable in 20‐nm‐wide In0.53Ga0.47As/In0.52Al0.48As multiquantum wells. The observed wavelength range was 0.8–1.8 &mgr;m. A proper choice of electric fields in the well on the order of 90‐170 kV/cm made it possible to magnify the photocurrent difference spectrum by 1×104, owing to the Stark shift of about 20–100 meV. The interband optical transitions were limited to the field‐sensitive ones of mostly forbidden type. As a result, conduction‐subband eigen energies in the wells were determined at room temperature. ©1996 American Institute of Physics.