Cloud condensation nucleus concentration (CCN) was measured directly at a supersaturation of 0.5% with a thermal diffusion cloud chamber at Cape Grim, Tasmania during the ACE-1 field study in November and December of 1995. Number-size distributions N(Dp) from 3 nm to 10,000 nm diameter were measured concurrently at the same location. The hygroscopic properties of the aerosol in the 50 to 300 nm diameter range (which contains most of the CCN active at 0.5%) were also measured. This data set provides a basis for investigating the relationship between measured and modeled CCN concentrations over an extended, four week period. The hygroscopic growth data were used to derive a ratio of droplet critical diameter to particle dry diameter for a supersaturation of 0.5% as a function of dry particle size. This empirically derived ratio incorporates the effects of soluble to insoluble mass as well as effective Van't Hoff factor and surface tension as described by Köhler theory for heterogeneous nucleation. The size distributions were integrated for dry diameters greater than the critical value and compared to the directly measured CCN concentrations. A significant fraction of the CCN at 0.5% derive from the seasalt mode with diameters, Dp>200 nm. The remainder are from the Aitken mode, Dp < 70 nm and the accumulation mode 70 nm < Dp < 200 nm.