Aflatoxins are toxic metabolites produced by Aspergillus fungi. They are widely recognized as a contamination for grains. Among the 20 carcinogenic natural aflatoxins, aflatoxin B₁ (AfB1) is considered the most toxic to animals and humans. Bentonite clays are used as anticaking additives in the pelletization of animal feeds. They also have shown extra benefits in reducing the concentration of AfB1 by adsorption. The major mineralogical and chemical properties of bentonites in determining their adsorption capacities for AfB1 are still poorly understood, which limits the selection, modification, and application of the clays as an aflatoxin binder. In this study, adsorption of AfB1 on clay fractions of six bentonites (referred to as 37GR, 1MS, 5OK, 7AZ, 8TX and 16MX) was investigated to determine the critical mineralogical properties as a good adsorbent. AfB1 adsorption isotherms were fitted with Langmuir, modified Langmuir with adsorption dependent affinity, and exponential Langmuir models. The interlayer cations in 37GR were replaced with four monovalent cations (Li, Na, K, Cs) and four divalent cations (Ca, Mg, Ba, Sr). The divalent cation saturation in general resulted in much higher adsorption capacity and affinity. Cations with small hydration radii lowered the affinity and adsorption capacity of the clay for AfB1. For all six clays, Ba saturation enhanced the size and polarity matching among clay particles, AfB1 molecules and cations, therefore increased the adsorption when compared with Ca saturation. A negative correlation was observed between cation exchange capacity and AfB1 adsorption. The importance of size and polarity matching in the adsorption process will be further verified in charge reduction experiments.