Cloud condensation nucleus concentration (CCN) was measured directly at a supersaturation of 0.5% with a thermal gradient diffusion cloud chamber at Cape Grim, Tasmania, during the First Aerosol Characterization Experiment (ACE 1) field study in November and December of 1995. Number-size distributions N(Dp) from 3 to 800 nm diameter and the hygroscopic properties of the aerosol in the 30 to 300 nm diameter range (which contains most of the CCN active at 0.5%) were measured concurrently at the same location. This data set provides a basis to compare measured and modeled CCN concentrations. A critical particle diameter that would form cloud droplets at 0.5% supersaturation was derived from the hygroscopic growth data including consideration of the hydration of the size distribution measurement. This empirically derived diameter incorporates the effects of soluble and insoluble mass as well as an effective van't Hoff factor and surface tension as described by K¿hler theory for heterogeneous nucleation of cloud droplets. The size distributions were integrated for diameters greater than the critical value and compared to the directly measured CCN concentrations. The modeled CCN concentration was 95 cm-3 during baseline sector periods and 128 cm-3 overall. This was about 20% greater than the directly measured CCN concentration and well correlated (R2=0.7) with measured CCN. Two thirds of the CCN at 0.5% supersaturation derive from an accumulation mode (80 nm200 nm) and the Aitken mode (Dp<80 nm). The data include air masses from continental, anthropogenic, and biomass burning sources as well as the more dominant marine source. ¿ 1998 American Geophysical Union