The thermodynamic quantities for the complex formation between the enantiomers of the beta-blocking drug alprenolol and cellobiohydrolase I (CBH I), that earlier has been used as a chiral selector for aminoalcohols, revealed positive Delta H-0 - values in all cases implying an entropy driven process. Association constants (K-a) for cellulase and the (R)- and (S)-enantiomers of alprenolol were determined by isothermal titration microcalorimetry and the inhibition constants (Ki) by enzyme inhibition experiments. Both inhibition experiments and microcalorimetry revealed that the affinity between the enantiomers of alprenolol and CBH I was higher in sodium phosphate buffer than in potassium phosphate buffer. This result was in agreement with previously reported liquid chromatographic separations of enantiomers using a chiral stationary phase based on CBH I immobilized to silica particles. The best fit of the isothermal titration data corresponded to a 1:1 binding isotherm.