[en] In this work, we report on observation of the effects of spatial quantization in very thick (up to 800 nm) GaAs layers confined with the AlGaAs barrier layers. We studied reflectance spectra of a series of hetero-structures with the quantum wells' thickness varying from 270 to 800 nm. The spectra were measured at 10 K in the spectral range 1500-1600 meV. It was found that the reflectance exhibits a quasi-periodic modulation associated with the quantum-size effect. The detection of the quantum-size effect in so thick layers indicates high quality of our heterostructures