This paper reports arsenic methylation in betaine-nontronite clay-water suspensions under environmental conditions. Two nontronites (<0.05 mm), NAu-1 (green color, Al-enriched) and NAu-2 (brown color, Al-poor, contains tetrahedral Fe) from Uley Mine - South Australia were selected for this study. Betaine (pK(a) = 1.83) was selected as methyl donor. The reaction between 5 g L-1 clay, 20 ppm As(III), and 0.4 M betaine at 7 <= pH(0) <= 9 under anoxic conditions was studied. The presence of nontronite clays were found to favor As(III) conversion to monomethylarsenic (MMA). Arsenic conversion was found to be as high as 50.2 ng MMA/ng As(111)(0). Conversion of As was found to be more quantitative in the presence of NAu-2 ((Na-0.72) Al0.16Fe0.29][Al-0.34 Fe-3.54 Mg-0.05] O-20(OH)(4)) than NAu-1 ((Na-1.05) [Si-6.38 Al0.95Fe0.07][Al-0.36 Fe-3.61 Mg-0.04] O-20(OH)(4)). The inherent negative charge at the nontronite tetrahedral layer stabilizes positively charged organic intermediate-reaction species, thereby leading to decreases in the overall methylation activation energy. The outcome of this work shows that nontronite clays catalyze As methylation to MMA via non-enzymatic pathway(s) under environmental conditions. (C) 2010 Elsevier B.V. All rights reserved.