In order to grasp tar reduction by partial producer gas oxidation, a two-step model is developed for computations on the chemical kinetics of partial oxidation. Free radical concentrations, and their residence times during partial oxidation are studied as a function of the gas hydrogen concentration. The computed free radical concentrations show to increase by an order of magnitude, and the residence times by three orders of magnitude, by increasing the producer gas hydrogen content. Further computations show that the concentrations and length of formed carbon containing molecules are largely independent of gas hydrogen content or air-excess ratio, but highly dependent on the gas C/O-ratio. (c) 2005 Elsevier Ltd. All rights reserved.