Abstract: Promising semiconductor devices, such as quantum-cascade lasers and similar to them heterostructures with multiple strongly-coupled quantum wells, may contain hundreds and even thousands of layers. Independent methods of photoluminescence (PL) and transmission electron microscopy (TEM), along with X-ray diffractometry and reflectometry, make it possible to characterize super-multiperiod (SMP—superlattices with 100 and more periods) structures and to determine, with high accuracy, the thicknesses of layers, the roughness/diffusion of interfaces and the composition of solid solutions. The difference between the expected and measured using PL and TEM thicknesses of the layers of GaAs/AlGaAs samples synthesized by molecular beam epitaxy was 5–10%. The SMP structures are characterized by stable thicknesses of the layers in depth and sharp boundaries with a width of transition interfaces of the order of several Å that is consistent with the X-ray analysis. Based on the data obtained on thicknesses of layers, by comparing the theoretical and experimental values of photoluminescence intensities it is possible to accurately determine the composition of Al_0.3Ga_0.7As which corresponds to the X-ray diffraction data.