The reaction of methyl benzoate [1] with NH3 was studied over Montmorillonite K10 clay in the temperature range 280-340 degrees C. Benzonitrile [2], benzoic acid [3], benzamide [4], N-methylbenzamide [5], and methanol were formed as products in varying amounts. This reaction was investigated as a function of molar Patio of 1 to NH3, temperature, and contact time. At 1/NH3=0.5, 320 degrees C, and contact time of 1.96x10-(3)h the conversion of 1 was 64.7% and the selectivities of 2 and 4+5 were 59.6 and 20.2, respectively. The clay catalyst was equally active for other aromatic and aliphatic esters. At 340 degrees C and 67.2% conversion of ethyl propionate the selectivity to propionitrile was as 73.3 after 4h reaction. The actual pathways leading to formation of products was studied in detail. ESCA, NK3 adsorption, pyridine adsorption, and IR studies, in combination with the catalytic investigation show that NH3 adsorbed on the acidic >AI-OH sites forms ammonium ions [NK4+] which participate in the reaction. The Lewis acid sites are likely to be involved in the formation of 2. The clay catalyst undergoes deactivation on continuous use for several hours [23h]. The deactivation is mainly due to dealumination by water produced in the reaction and accumulation of organic residue containing an amide group on the surface of the clay. It was also possible to calculate effective active sites on the catalyst surface and then a correlation with the turnover of formation of benzonitrile could be established.