Alkylation of diphenylamine was carried out with alpha-methylstyrene and diisobutylene using acid-treated clay catalyst Engelhard F-24 to produce mono- and dialkylated diphenylamines. Dialkylated diphenylamines are industrially useful as antioxidants and head stabilizers. Mono and dialkylated diphenylamines were separated in pure form from the reaction mixture and characterized by FTIR and H-1 NMR. Reaction conditions for obtaining high selectivity of each of the alkylated products were studied. A comparative study of the reaction rates, with various other grades of acid-treated clay catalysts, was done for the reaction between diphenylamine and alpha-methylstyrene. The reaction between diphenylamine and alpha-methylstyrene was also studied in a continuous mode of operation ire a fixed bed reactor. Deactivation of the catalyst occurred during the course of these reactions and reuse of the catalyst posed problems; alternative schemes were considered to regenerate the deactivated catalyst. The basic reasons for the deactivation of the catalyst were investigated in the case of reaction between diphenylamine and alpha-methylstyrene. The fresh and used catalysts were characterized using various spectroscopic and other techniques such as X-ray diffraction, X-ray photoelectron spectroscopy, UV-visible and nuclear magnetic resonance spectroscopy, surface area (BET) and surface acidity (TPD), pore size distribution, and pore volume. These studies indicated that the deactivation of the catalyst was due to irreversible adsorption on the surface, loss of microporous structure and loss of surface acidity The adsorbed species appeared to be diphenylamine and/or alkylated product(s), coordinated to the surface oxygen of the catalyst through nitrogen, forming a nitro kind of species.