This study focuses On the effects of flame stretch on the mass burning rate, for flames with nonunit Lewis numbers. The extended flame stretch model of de Goey and ten Thije Boonkkamp [Combust. Flame 119 (1999) 253-271], which was derived for multiple-species transport and chemistry, is used as a starting point. This model is adjusted to be able to predict the mass burning rate at the inner reaction layer of the flame. The accuracy of the theory is analyzed step by step using numerical results incorporating detailed chemistry and multispecies transport models. The adjusted model proves to be an accurate predictor of the mass burning rate. Furthermore, it is shown that not only the lean species Lewis number plays a role but also a number of other Lewis numbers have a significant influence on preferential diffusion. Preferential diffusion for methane/air mixtures is more difficult to predict accurately than for ethane/air and propane/air mixtures. Different contributions to the total preferential diffusion effect in methane/air mixtures partly cancel, which corresponds to an effective Lewis number being close to one. For fuels with Lewis numbers further away from one. no cancelation takes place and the accuracy of the predicted mass burning rate increases. Results show that the extended theory forms the basis for the first quantitative model to predict mass burning rates of premixed, laminar, stretched flames with nonunit Lewis numbers. (c) 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved.