It has been frequently hypothesized that the presence organic films can influence the activation of cloud condensation nuclei (CCN) to form cloud droplets by inhibiting condensational growth. A variety of past observations have suggested that such films may exist at the aqueous/air interface for atmospheric aerosol. Laboratory experiments on model aerosols with single compound films have measured mass accommodation coefficients (sometimes termed condensation coefficient) as low as 10^-5, values that are sufficiently small to strongly inhibit condensational growth and therefore cloud droplet formation. A novel experimental study using a modified tandem DMA to directly measure the potential influence of organic films on aerosol growth rates has been performed and will be described. This appears to be the first experiment to directly measure the time scale for condensational growth for the size range of aerosols relevant to cloud formation. Results from field tests in Mexico City during autumn 2000 during both the dry and wet seasons will be analyzed and presented. Parallel measurements of aerosol chemical composition and surface tension will contribute to the analysis. Implications for the indirect effect of aerosols on climate will be discussed.