Delivering electron acceptors, electron donors, and nutrients in gas state has been practiced for in situ bioremediation. A mathematical model based on air-channel concept was developed in this paper to assess the dissolution transient behavior of gaseous substrates for an in situ sparging process using their chemical and physical properties, aquifer-media characteristics, and field operating conditions. Using toluene as an example. the model was verified with experimental data obtained at field sparging rates ranging from 40 to 80 L air/min. The verified model is a useful means of predicting the dissolution behavior of gaseous substrates during sparging in an unconfined aquifer. (C) 2008 Elsevier B.V. All rights reserved.