The use of bentonite for recycling vegetable oils presents challenges, as even minor variations in the clay composition and structure can lead to significant differences in its ability to retain various chemical groups. This study investigates the structure-activity relationship of four bentonites-two hydrophilic and two hydrophobic (both in commercial and ground forms)-to better understand these effects. Solid-state NMR spectroscopy revealed subtle differences between hydrophobic and hydrophilic materials, as well as distinctions between ground and unground hydrophilic clays, through