Aerosols are prevalent in the atmosphere where they can serve as cloud condensation nuclei (CCN). Here we report on new CCN measurements to characterize the temperature dependence of CCN activity for single component organic aerosol and secondary organic aerosol from the reaction of alpha-pinene and O-3 generated in the laboratory. For compounds with a weak dependence of water activity on temperature, the critical supersaturation of the particles can be well-modeled using temperature-dependent surface tension and Kohler theory. We also demonstrate that Kohler theory is valid for sparingly soluble compounds and can quantitatively explain the temperature-dependent activation properties of adipic acid aerosol over a broad range of temperatures. Accounting for temperature-dependent surface tension strongly affects CCN activity, and we anticipate that including this effect in global and regional model simulations may significantly change the estimated aerosol indirect forcing.