BACKGROUND: Reactions performed under solvent-free conditions give processes that are environmentally friendly, since most solvents are polluting agents. In this work, the performance of Candida rugosa lipae (CRL) immobilized on styrene-divinylbenzene (STY-DVB) or controlled pore silica (CPS), and the commercial lipase Novozym 435, was evaluated for the synthesis of butyl esters in solvent-free systems (SFS). A 2 2 full factorial design was used to study the influence of the organic acid chain length and the biocatalyst concentration on the esterification performance. RESULTS: When CRL on STY-DVB was used, the ester formation was influenced by both variables and their interaction. The reaction conversion was higher (63%) using 10% of immobilized system and lauric acid, corresponding to a productivity of 3.62 g L-1 h(-1) For CRL on CPS, only the effect of biocatalyst concentration was significant, and the highest yield was attained using 20% of immobilized system and caprilic acid. In the case of Novozym. 435, the highest yield (49%) was obtained using butyric acid as acyl donor at 15% of immobilized lipase. CONCLUSION: The results allowed better understanding of the influence of important parameters in this environmentally friendly process, which also has the process advantage of a higher volumetric productivity when compared with a solvent system. (c) 2007 Society of Chemical Industry.