An extension of the models developed by Guha and Jaffe (Biotechnol Bioeng [1996] 50:693-699) to describe the phenanthrene biodegradation kinetics for the cultures with variable fractional volumes is presented. Batch experiments were conducted with a culture capable of degrading the phenanthrene using a single culture vessel from which samples were withdrawn over time to monitor the disappearance of phenanthrene. For accurate measurement of phenanthrene concentrations, a sampling procedure designed for quantifying the sorption of phenanthrene onto glassware was also introduced. The Monod parameters were estimated by nonlinear regression analyses of simultaneous solutions to the substrate utilization/volatilization and Monod equations for growth of the cell mass. The results demonstrate that the models were able to be extended to phenanthrene-degrading cultures with variable fractional volumes. When the ratio between sampling volume and volume of the culture medium was relatively small, the parameters obtained were similar to those which would be obtained using constant fractional volumes of culture medium. It was also found that the model's fit to the phenanthrene disappearance data in this study were better than those obtained by Guha and Jaffe, implying that the sorption process of phenanthrene during the sampling period could significantly affect the measurement of phenanthrene concentrations. Failing to account for these losses led to less accurate measurements of substrate concentrations, which in turn resulted in a poor estimation of the parameters. The findings of this study reduce considerably the experimental work necessary in the estimation of Monod kinetic parameters for the purpose of modeling. (C) 2004 Wiley Periodicals, Inc.