Volatile properties of maritime and modified maritime aerosol were measured at the remote site at Mace Head (53¿19'N, 9¿54¿W) on the west coast of Ireland, located on the eastern edge of the North Atlantic Ocean. The volatility measurements were made with a light-Scatering counter equipped with a temperature-controlled heated inlet. The work extends the temperature range from 300¿C to 850¿C for the first time for aerosol voltatility studies. Representative measurements made over the period of about a year show that the submicrometer particles with radius <0.2 μm are highly volatile for the maritime aerosol and show temperature fractionation features of ammonium sulfate (or ammonium bisulfate). It is estimated that 85--95% of this size fraction (by volume) is composed of these sulfates. For the higher temperatures, temperature-fractionation characteristics of sodium chloride are shown for the supermicron and also the submicron maritime particles. About 80% of the particle size interval of 0.3--1.5 μm radius is composed of sodium chloride. The temperature profile curves for the modified maritime aerosol, which has made a partial tranverse overland, do not display definitive features characteristic of known atmospheric constituents such as ammonium sulfate or sodium chloride, but rather properties indicative of mixtures of these species with other unknown constituents. For both airmasses, between about 5% and 30% (by volume) of the aerosol particles remain involatile at least up to a temperature of 850¿C. ¿American Geophysical Union 1990