When an electric field F is applied parallel to the growth axis of a semiconductor superlattice, the eigenstates are quantized and strongly localized. This Wannier-Stark quantization manifests itself as an effective blue shift of the optical-absorption edge and is accompanied by a new type of electro-optical oscillation of the absorption coefficient which is periodic in F _1 . We report experimental evidences for these theoretical predictions from the study of the low-temperature electroreflectance of a GaAsAlGaAs superlattice.PACS numbers: 73.20.Dx, 78.20.Jq The nature of the eigenstates in a crystal in the presence of an external electric field F has been a controversial matter for many years. 1 " 3 On the one hand, it is predicted, in a simplified tight-binding approach, that the eigenstates are quantized "Stark ladder" states which are localized over a length X =AE/eF, where AE is the bandwidth and e the electronic charge. On the other hand, in the effective-mass approach, the eigenfunctions are Airy functions which are delocalized and correspond to a continuous spectrum. The two approaches are valid in different ranges of electric fields and crystal sizes, but none is fully satisfying: The effective-mass approach obviously breaks down when the total potential drop across the crystal is larger than AE, while the single-band tight-binding approach becomes at least suspect when the potential drop becomes larger than a band gap. In bulk materials there is no clear-cut experimental evidence of Wannier-Stark states, and, in contrast, the observed electro-optical properties are quite consistent with the predictions of the effective-mass theory, known as the Franz-Keldysh effect. 4 Recently, we investigated theoretically the electrooptical properties of semiconductor superlattices (SL) 5 which are unique systems where the bandwidth can by taylored in the range of a few tens of meV. This means that the high-field limit eFd/AE >. 1, where d is the SL period, can be explored with moderate fields in the range of a few tens of kV/cm. For such systems, we predict that, as the field is increased, the system behaves more and more like a series of uncoupled quantum wells (QW), and its optical-absorption line shape evolves from the miniband profile at F=0 to a step function at large field, showing a blue shift 6 of the absorption edge of the order of the half sum of the conduction (AE\) and valence (AH\ or AL\) miniband widths. The main step at the midsubband energy E^w is accompanied with smaller ones at Ep w +peFd (p = ± 1, ± 2,. . . ), corresponding to "oblique" transitions in real space connecting an electron localized near the nth quantum well with a hole in the (n+/?)th quantum well. Because of these oblique transitions, the absorption coefficient at a fixed energy oscillates with the electric field, and these oscillations are periodic in F~l. This F~x dependence is the signature of the evenly spaced Stark-ladder spectrum.In this Letter, we report the observation of these new effects by a study of the low-temperatur...