Rhodococcus erythropolis is a promising Gram-positive bacterium capable of numerous bioconversions including those involving alcohol dehydrogenases (ADHs). In this work, we compared and optimized the redox biocatalytic performances of 1-butanol-grown R. erythropolis NCIMB 13064 cells in aqueous and in non-conventional gas phase using the 1-butanol-hexanal oxidation-reduction as model reaction. Oxidation of 1-butanol to butanal is tightly coupled to the reduction of hexanal to 1-hexanol at the level of a nicotinoprotein-ADH-like enzyme. Cell viability is dispensable for reaction. In aqueous batch conditions, fresh and lyophilized cells are efficient redox catalysts (oxidation-reduction rate = 765 mu mol min(-1) g cell dry mass(-1)) being also reactive towards benzyl alcohol, (S)-2-pentanol, and geraniol as reductants. However, butanol-hexanal oxidation-reduction is strongly limited by product accumulation and by hexanal toxicity that is a major factor influencing cell behavior and performance. Reaction rate is maximal at 40 degrees C-pH 7.0 in aqueous phase and at 60 degrees C-pH 7.0-9.0 in gas phase. Importantly, lyophilized cells also showed to be promising redox catalysts in the gas phase (at least 65 mu mol min(-1) g cell dry mass(-1)). The system is notably stable for several days at moderate thermodynamic activities of hexanal (0.06-0.12), 1-butanol (0.12) and water (0.7). (C) 2008 Elsevier Inc. All rights reserved.