Kinetics of side reactions of the formation of n-butyl acetate in the heterogeneously catalyzed esterification of n-butanol with acetic acid were studied in an isothermal fixed-bed flow reactor at temperatures between 100 and 120 degrees C. The observed side reaction products are isomers of butene, di-n-butyl ether, sec.-butyl-n-butyl ether as well as sec.-butanol and sec.-butyl acetate. Three ion-exchange resin catalysts with a similar matrix but different sulfortation are compared: Purolite (TM) CT 269 is mono-sulfonated whereas Amberlyst (TM) 46 is surface-sulfonated and Amberlyst (TM) 48 is bi-sulfonated. Purolite (TM) CT 269 and Amberlyst (TM) 48 are fully sulfonated in the gel phase, whereas Amberlyst (TM) 46 is only surface-sulfortated. The ion-exchange capacities of Purolite (TM) CT 269 and Amberlyst (TM) 48 are similar, that of Amberlyst (TM) 46 is lower by a factor of 5. Despite this, all three catalysts show only minor differences in their activity regarding the esterification. Regarding the formation of side products, Purolite (TM) CT 269 and Amberlyst (TM) 48 give similar results: side reactions proceed to significant extent. For Amberlyst (TM) 46, however, side reactions were found to be almost negligible. It is concluded that the esterification occurs mainly on or near the external surface of catalysts particles whereas side reactions occur mainly in the pores. This work shows that surface-sulfortated catalysts like Amberlyst (TM) 46 are very attractive for the production of esters by reactive distillation. (c) 2005 Elsevier Ltd. All rights reserved.