Enzyme performance is often impaired in supercitical carbon dioxide. We were able to enhance enzyme activity in this,medium via changes in acid-base conditions by using ion-exchange materials (solid H+/Na+ buffer pairs and a zeolite), which were selected on the basis of the response of an organosoluble acid-base indicator. The concentration of ion-exchange materials had an important effect on the catalytic activity of subtilisin Carlsberg cross-linked enzyme crystals (CLECs), and this was related to the, protonation and hydration states of the enzyme. The buffer Na2CO3/NaHCO3 gave the highest enhancement in enzyme activity (by a factor of 54), probably a result of its, high basicity and capacity to counteract the deleterious effect of carbonic acid to a greater extent than the other materials tested.