The aromatic diketone 6R-dihydro-oxoisophorone (DOIP) is an important intermediate in the synthesis of some naturally occurring carotenoids. Its preparation via the reduction of 4-oxoisophorone (OIP) by baker's yeast has previously been developed to a pilot-scale process. In this work, the kinetics of substrate inhibition and of product degradation during the reduction of OIP using resting baker's yeast cells as catalyst is studied. Substrate inhibition during the reduction can be described by a non-competitive type of inhibition. Product is degraded to an unwanted by-product 4S,6R-actinol by baker's yeast. This reaction can very well be described by a second-order rate equation with respect to DOIP concentration, which is an exceptional case for a whole-cell-catalyzed reaction system. (c) 2005 Elsevier Inc. All rights reserved.