The ratio of initial substrate concentration (SO) to initial biomass concentration (X-0) represents the availability of carbon and energy source for microbial growth in a batch culture. Based on the concept of consecutive catabolism and anabolism, a thermodynamic approach was used to derive the expression of the specific growth rate to S-0/X-0, which shows that microbial growth kinetics are determined by the S-0/X-0 ratio in batch culture, while the constants involved in the proposed model have clearly defined physical meanings. The proposed model was verified by the experimental data obtained at different S-0/X-0 ratios, while literature data also supported the model derived. This study provides theoretical evidence that the S-0/X-0 ratio is the most important determinant of kinetic ability of microorganisms in a batch culture. When batch culture is chosen to evaluate the kinetic parameters for process design and ready biodegradability of substances, care is required in designing initial conditions of batch culture since microbial growth kinetics is highly influenced by the S-0/X-0 ratio. (C) 2004 Elsevier Ltd. All rights reserved.