Simultaneous air samples were taken in Chicago and over southern Lake Michigan as part of the AEOLOS Project (Atmospheric Exchange Over Lakes and Oceans). Gas and particle phase concentrations of polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and total suspended particles (TSP) were measured over 12 h periods during July, 1994, and January 1995. Partitioning of PCBs and PAHs between gas and particle phases was well correlated with the subcooled liquid vapor pressure (p°(L)) for individual samples, but the relationship differed among samples. For all but a few of the samples the slopes of the log K(p) vs log p°(L) lines were statistically greater than -1. Other investigators who have found similar results have concluded that the PCBs/PAHs were not an equilibrium; however, other factors indicate that the PCBs and PAHs in the Chicago/Lake Michigan atmosphere are at equilibrium. Slopes of the regression of log K(p) vs log p°(L) from samples of continental background origin, and therefore assumed to have had sufficient atmospheric residence times to reach equilibrium, are among the shallowest measured (-0.70 to -0.53 and -0.16 to - 0.56 for PAHs and PCBs, respectively). One pair of samples where the air mass is believed to have been sampled twice, once in the urban area and again ~3.4 h downwind, shows no difference in partitioning. PCBs and PAHs measured in Chicago and over Lake Michigan were apparently at equilibrium between the gas and particle phases. A slope of -1 in the regression of log K(p) vs log p°(L) is not necessary to describe equilibrium partitioning. Differences in particulate matter may be responsible for the shallow slopes observed.