The low-temperature direct and indirect optical transitions in double-barrier Al0.3Ga0.7As/AlAs/GaAs single quantum wells, with AlAs widths of 0, 1, and 2 monolayers, have been studied as a function of pressure. It is shown that intense type-II recombinations occur under pressure in AlAs-bordered wells, due to X electron localization in the wells formed by the ultrathin AlAs layers. A fairly accurate description of the luminescence line energies and relative intensities is obtained over the whole pressure range investigated, with use of the envelope-function formalism.