Dense gases were used as a biochemical reaction medium. Esterification of oleic acid with oleyl alcohol, catalyzed by lipase from Rhizomucor miehei (Lipozyme(IM)) was used as a model system. Due to the limitation of the process that may arise from the non-polarity of carbon dioxide, which preferentially dissolves hydrophobic compounds, studies were also performed with other gases (n-butane, n-propane, n-propane/n-butane mixture). The study of the pressure stability of the immobilized lipase showed that the lipase is quite stable; it does not lose its activity when it is exposed to various dense gases at high pressure for a longer time. Esterification rates at high pressure were determined for all systems, and it was found that they were higher than at atmospheric pressure. The studies of thermodynamic properties and mass transfer were performed in a batch stirred tank reactor in dense carbon dioxide for the esterification of oleic acid with oleyl alcohol (catalyzed by lipase). The highest rate and maximum conversion were determined. It was found that in a continuous fixed bed reactor at 150 bar, 40 degrees C and water activity 0.46% w/w, the activity of the enzyme preparation was practically unchanged when CO, was used as a solvent. The addition of small amounts of water increases the conversion rate. A higher conversion was also observed at a longer residence time. When n-butane was used as a reaction medium a decrease of conversion was observed.