Accurate knowledge of Henry's law constants (K_aw) is very important for fate and transport studies of organic chemicals. Here field-derived K_aw values for polychlorinated biphenyl (PCB) congeners (PCBs 28, 49, 52, 118, 138, 149, and 153) were determined in the open Atlantic Ocean, at locations where air and seawater were assumed to be at equilibrium. Field-based Kaw values were derived from air and seawater samples simultaneously collected at these locations, and their relationship with temperature was determined. The average field-derived K_aw for PCBs 28, 49, 52, 118, and 138 was generally 2-3 times higher than laboratory-derived values reported in the literature. Possible causes of differences between the field and the laboratory data may be attributed to differences between pure water and seawater and the use of much higher persistent organic pollutant concentrations in laboratory studies compared to that of the remote ocean. Field-derived K _aw values for PCB 149 and PCB 153 were up to 10 times higher than literature values. The comparison of field- and laboratory-derived K _aw shows that their temperature dependence is not significantly different (95% confidence) from those reported in the literature.