AB: Modeling studies hint at the large impact of competition between pre-cloud aerosol of varying compositions and sizes on cloud condensation nuclei concentrations (e.g., Ghan et al., J.~Atmos.~Sci., 55(22), 3340, 1998). In certain situations, the presence of a small number of favorably composed aerosol is as important to aerosol activation as the total aerosol loading. Yet most microphysical models used to investigate aerosol activation resolve too poorly the aerosol mixing state and variation amongst the particles. Also, large-scale investigations into the indirect effects generally parameterize activation as a simple function of sulfate loading, and so miss these dynamics entirely. We investigate the sensitivity of aerosol activation to mixing state of inorganic aerosol populations using a new aerosol-cloud microphysical model that fully resolves both size and composition. The model incorporates aerosol thermodynamics, aqueous and gas phase chemistry, and a non-equilibrium condensation schemes. We present the results of initial studies into how the activation statistics of aerosol in simple externally mixed inorganic systems change when: a).~variations in composition amongst similarly sized particles are increasingly resolved; and b).~a small number of favorably composed particles are introduced. We suggest further studies that will lead towards parameterizations of multi-component and externally-mixed aerosol activation. These parameterizations should be suitable for representing the relationship between mixed aerosol populations and cloud condensation nuclei formation necessary for including the indirect effect in larger-scale models.