An intensive field sampling program has been carried out over the ocean in which 56 gas samples and 172 aerosol particle samples were analyzed for total organic chlorine and total inorganic chlorine gas and for particulate chlorine as a function of particle size. Sampling was conducted for 13- to 21-hour periods, March 5--14, 1976, 4 km from the north Florida shore of the Gulf of Mexico at 10-m and 2-m heights. Replicate samples collected simultaneously showed agreement within analytical errors generally of a few percent for gases and for the sum of particle size fractions. Over the course of the experiment, 21 ambient measurements of organic chlorine averaged 3050 ng Cl/m3 STP of air, with a standard deviation of the distribution of measurements of 80 ng Cl/m3 STP. Also in 21 measurements the sum of particulate and inorganic chlorine averaged 3030¿190 ng Cl/m3 STP. Thus the total chlorine concentration observed averaged 6070¿220 ng Cl/m3 STP, of which 50% was organic gaseous. In contrast, the proportion of inorganic gaseous chlorine varied drastically, ranging from the one half to one hundred times the observed particulate chlorine concentration. The variation depended at least in part on whether cascade impactors with Nuclepore backup filters or filters alone were used for particle samples before the airstream passed through treated LiOH-impregnated gas absorption filters and charcoal absorbers. The results suggest that rapid interchange occurs between inorganic gaseous and particulate chlorine in the atmosphere but that organic gaseous chlorine is relatively decoupled from these forms.