The mechanism of the hydrodenitrogenation of the mixed dialkyl- and trialkylamines C-1-NH-C-6 and C-1-N(C-6)(2) was studied over sulfided NiMo/gamma-Al2O3 at 280 degrees C and 3 MPa. C-1-NH-C-6 reacted by disproportionation to C-1-N(C-6)(2) as well as C-6-N(C-1)(2) and by substitution by H2S to methylamine and hexanethiol as well as hexylamine and methanethiol. C-1-N(C-6)(2) reacted by substitution with H2S to C-1-NH-C-6 and C-6 NH-C-6 and methane- and hexanethiol. The probability of breaking the C-1-N bond was only slightly smaller than of breaking the C-6-N bond in C-1-N(C-6)(2). In the reaction of an equimolar mixture of C-5-NH-C-5 and C-1-N(C-6)(2) both C-1-N(C-5)(2) and C-6-N(C-5)(2) were formed. The transfer of the methyl group in these reactions cannot be explained by imine and enamine intermediates, only iminium cation intermediates can explain all the products in the hydrodenitrogenation of monoalkyl-, dialkyl- and trialkylamines. (C) 2006 Elsevier B.V. All rights reserved.