The deposition of respiratory aerosols during the expiratory phase of breathing may be significant. However, only a limited number of studies have considered the effects of exhalation on aerosol deposition in bifurcating respiratory geometries. In this study, double bifurcation models of respiratory generations G3-G5 and G7-G9 were used to determine the deposition of aerosols in the size range of 1-7 mu m during steady exhalation. The geometries considered were based on standard and 30% constricted pediatric airway models. A previously tested CFD code was implemented to predict aerosol deposition under laminar flow conditions. Results indicated that deposition in bifurcating airways during expiration was a function of the Stokes number, which represents particle inertia, and the Dean number, which captures the strength of secondary flow conditions. For a Dean number of approximately 10, deposition was observed to decrease with increasing values of the Stokes number. Increasing the Dean number above approximately 100 resulted in a significant increase in deposition for constant Stokes number values. Existing algebraic correlations did not capture the effects of the Dean number on deposition during exhalation. As a result, novel correlations were proposed that predicted branch-averaged deposition efficiency as a function of both Stokes and Dean numbers. implementation of these correlations into whole-lung models is suggested to improve the prediction of deposition during exhalation under laminar expiratory conditions in bifurcating airways. (C) 2008 Elsevier Ltd. All rights reserved.