Environmentally benign Bronsted acidic ionic liquids with SO3-H functionality were prepared using N-methyl imidazole and 1,3-propane sultone as the source chemicals. The prepared ionic liquid catalysts were characterized by IR, NMR, TG and their catalytic activity in esterification of butanoic acid with butanol was investigated. An experimental design using response surface methodology (RSM) was used to optimize the process parameters in esterification to minimize rigorous experimental procedures and conserve the catalyst. The parameters, namely, reactant mole ratio, amount of catalyst, time and temperature of reaction on the conversion of butanoic acid were optimized using Box-Behnken design. The obtained optimum conditions were n (butyric acid):n (butanol)= 1:1.6, amount of catalyst of 5 wt% to butanol, reaction time 2.5 h and reaction temperature 117 degrees C. Under the optimized conditions, the yield of butyl butyrate reached 98.41%, in close agreement with values predicted by the mathematical model. The ionic liquids could be reused five times without noticeable drop in activity. (C) 2011 Elsevier B.V. All rights reserved.