Organic species in sea salt particles can significantly reduce hygroscopic growth in subsaturated conditions, an important uncertainty in the radiative effect of aerosol particles on the atmosphere. This hygroscopic behavior is predicted with a numerical model of the organic-water, electrolyte-water, and organic-electrolyte interactions in complex mixtures of organic species and inorganic ions. The results show a 15% decrease in hygroscopic growth above 75% relative humidity for particles that include as little as 30% organic mass. Organic compositions of 50% organic mass reduce hygroscopic growth by 25%. This prediction relies on particle chemical composition estimated from measurements of insoluble organic species in marine-derived particles and of soluble organic species measured in seawater. Twenty insoluble and four soluble organic species are used to represent the behavior of sea salt organic composition. The hygroscopic growth is strongly sensitive to the organic fraction that is soluble or slightly soluble, although variations among different soluble or insoluble species are small above the sodium chloride deliquescence point. Interactions between organic and electrolyte species depend primarily on the salting out behavior of NaCl with alkanes, carboxylic acids, and alcohols, although interactions with other inorganic ions in sea salt were estimated to cause small changes in the hygroscopic growth. The predicted growth factors for sea salt with <30% organic species are consistent with growth factors measured for ambient marine-derived particles by another group <Berg et al., 1998; Swietlicki et al., 2000; Zhou et al., 2001>. This coincidence suggests that the less-hygroscopic particles could indicate the presence of marine organic compounds, although multiple combinations of inorganic and anthropogenic organic species would also satisfy the measured behavior. ¿ 2001 American Geophysical Union