The biomass growth, substrate consumption and polyhydrobutyrate (PHB) production of Ralstonia eutropha with butyric acid and fructose as the carbon and energy sources at various ratios of initial substrate concentration (S-0) to initial biomass concentration (X-0) were investigated in this study. Results indicated that the PHB content increased with the increasing S-0/X-0 ratio. Different substrates exhibited a similar trend for cell growth and substrates consumption with the changing S-0/X-0 ratio. The specific consumption rates of both butyric acid and fructose increased with the increasing S-0/X-0 ratio. An S-0/X-0-dependent kinetic model was modified to describe the kinetics of biomass growth and substrate consumption for R. eutropha. This model was verified with the experimental results from this work and in literature. (c) 2006 Elsevier Ltd. All rights reserved.