Interactions between solute and solvent in supercritical fluids (SCFs) are of fundamental importance because small changes in pressure or temperature near the critical point may alter reactivity in chemical and biochemical processes, and were examined such as those for ester synthesis catalyzed by a lipase in supercritical carbon dioxide and for a FT-IR study on the structures of reverse microemulsions in supercritical ethane. We previously conducted ester synthesis from acyl donors and terpene alcohols catalyzed by Candida cylindracea (CCL) lipase in supercritical carbon dioxide. In the near-critical region, (S)-(-)-terpene esters were stereoselectively synthesized from acyl donors and a primary alcohol such as (+/-)-citronellol. In a very limited pressure range near the critical point, interactions between carbon dioxide and enzyme molecules greatly increased with consequent drastic conformational changes in the enzyme, causing active sites to emerge to catalyze stereoselective synthesis. Therefore, supercritical carbon dioxide medium in the near-critical region should trigger the activation of the enzyme by causing movement of its surface groups and creating active sites. Two molecular aggregates, an enzyme and micelle, in supercritical fluids were studied with respect to their microstructure and activity for chemical reactions. High-pressure FT-IR spectroscopy has thus been used to clarify (i) the rotational isomerism of AOT molecules, (ii) characteristics of water and the water-head group, and (iii) RSO3-Na+ interactions in AOT reverse micellar aggregates in supercritical ethane. At 35.0 MPa, the proportion of trans-like to gauche-like conformers greatly increased as W0 was increased. A system of water/AOT/supercritical ethane was in a one-phase microemulsion state at W0 values less than 15 at 35.0 MPa and 306.1 K. At higher W'0 the system was in a two-phase state. The addition of lithium chloride makes the system become motionless in a one-phase microemulsion state at W0 above 30. This work demonstrates interesting pressure-, temperature, and salt effects on an enzyme-catalyzed esterification and/or maintenance of a one-phase microemulsion in supercritical fluids from practical and theoretical points of view.