Heterogeneous synthesis of 3,3'-dimethyl-4,4'-diaminodiphenylmethane (MDT) from o-tolylamine and formaldehyde on acidic zeolites HY, HZSM-5, Hp and TS-1 was investigated. H beta zeolite with strong Bronsted acidity and proper textural property demonstrated better catalytic performance than other three zeolites for this synthetic reaction. Yield and selectivity of product MDT from the catalytic reaction over H beta strongly depended on temperature, and optimal reaction temperature appeared to be about 433 K. Based on analyses of LC/MS and H-1 NMR for reaction intermediates, it was suggested that the synthetic process went through multiple steps involving intermediates o-tolylamino-methanol, methylene-o-tolylamine, N,N'-di-o-tolyl-methanediamine and 2-methyl-4-(o-tolylamino-methyl)-phenylamine. Interpretation to the mechanism of various intermediates formation and conversion was presented. (C) 2014 Elsevier B.V. All rights reserved.