Models incorporating liquid-gas mass-transfer and biofilm mass-transfer resistances were developed for trickling filters (TF) and rotating biological contractors (RBC). Biodegradation and volatilization coefficients were estimated from the previously described pilot-scale studies. The volatilization coefficients of a given compound in the TF and RBC were generally constant across the experimental conditions investigated. While biodegradation-rate coefficients were constant in the TF across experimental conditions, in the RBC the biodegradation-rate coefficients appeared to be greatest under conditions of low loading and high disc rotation speed and lowest under conditions of high loading and low disk rotational speed. The biofilm was completely penetrated by most of the contaminants and diffusional resistance did not limit the rate of biodegradation of any of the compounds. In the RBC, diffusion in the biofilm appeared to be limiting the biodegradation of toluene, o-xylene, and 1,3,5-trimethylbenzene. The ratio of gas- and liquid-phase mass-transfer coefficients ranged from 91.4 for the TF to 5.6 for the RBC. Due to the relatively wide confidence intervals associated with these estimates, the values could not be statistically differentiated, however, the results suggest a significant contribution of gas-phase resistance to mass transfer in some cases.