Ethyl (S)-3-hydroxybutyrate ((S)-E3HB) is naturally found as part of kiwi fruit aroma and is an important precursor for the production of several pharmaceuticals. (S)-E3HB can be produced in a whole cell biotransformation process from Ethylacetoacetate (EAA) using the yeast Saccharomyces cerevisiae. One way of improving the sustainability of such processes may be their integration into novel biorefinery concepts, which requires research to characterize the process and the byproduct formation. In this study it could be shown that (S)-E3HB biocatalysis with S. cerevisiae can be performed under anaerobic conditions with production rates between 4.0 × 10-3 and 2.5 × 10-2 g(S)-E3HB gbiomass-1 h-1 which are similar to the rates in conventional aerobic processes. The byproduct spectrum allows for the integration of the new anaerobic biotransformation process into a recently invented biorefinery concept in which the side product CO2 is further utilized to produce methane.