Substantial losses of aerosol particles can occur in the inlet region of isokinetic sampling probes. Okazaki and Willeke developed a semiempirical model based on gravitational settling in developing boundary layers to predict these losses. However, that model predicts zero losses in vertical probes, which is contrary to experimental evidence. From first principles, we have numerically analyzed depositional losses in isokinetic probes by calculating the flow field and then determining particle trajectories. By including the Saffman force (which causes motion transverse to the free stream velocity direction) in addition to the gravitational force, we have shown good agreement with the semiempirical model of Okazaki and Willeke. The geometric standard deviation is 1.16 for our calculations relative to the Okazaki-Willeke model for a range of conditions which they tested. In comparison, the geometric standard deviation is 1.12 for the experimental data of Okazaki and Willeke relative to their semiempirical model. © 1992, American Chemical Society. All rights reserved.