Our previous study has demonstrated that industrial waste biomasses of Penicillium oxalicum var. Armeniaca and Tolypocladium sp., could be used for biosorption of Cd, Pb, Hg and Cr(VI). The objective of the present study was to investigate the biosorption mechanisms, while using FT-IR spectroscopy and SEM technique. SEM microscopic observations (coupled with EDX analysis) confirmed the presence of the target metals on the biosorbents surface. Carboxyl groups were identified by FT-IR spectra analysis on the surface of Tolypocladium sample, while the alkaline treated Penicillium sample was characterized by the presence of amine groups. It was proved that the metal biosorption slightly modified the FT-IR spectra, while the mentioned functional groups were involved in the uptake mechanism (chelation on nitrogen containing groups and ion exchange on carboxyl groups).