We have determined the elemental and the stable chlorine isotopic composition of size-segregated aerosols collected in the marine boundary layer. All size fractions have chlorine deficiencies. Mid-size aerosols are isotopically enriched in Δ37Cl and contain significant quantities of non-sea-salt-SO42- and NO3-. This is consistent with the acid displacement of HCl from the aerosols. Coarse aerosols tend to be isotopically depleted and have Cl deficiencies that are not charge balanced by MSA-, SO42- and NO3- with Cl volatilization by a different mechanism. The isotope systematics imply a fractionation factor, &agr;, of 1.00306¿0.0005 if the volatilization mechanism is first order with respect to Cl or 1.00280¿0.0005 if the volatilization mechanism is zeroth order with respect to Cl. Our data are consistent with the efficient recycling of aerosol Cl through the gas phase. ¿ 1998 American Geophysical Union