The rate of condensational growth is relevant to a variety of atmospheric processes, in particular the activation of cloud condensation nuclei to form cloud droplets. It has been hypothesized that the presence of organic films on the surface of aerosols can strongly inhibit condensational growth, but few relevant observations of atmospheric aerosol growth rates appear to have been previously reported. A brief review of the literature on the effects of films on the mass transfer of water is presented. A novel experiment based on tandem differential mobility analysis for the measurement of the timescale for condensational growth (τg) for ambient aerosols is introduced. Measurements were taken during the wet season (25--27 September 2000) and dry season (16--17 November 2000) in Mexico City for particles with diameters of 50 and 100 nm. The results show that most of the particles exhibit τg < 2 to 3 s. However, a small number (between 0 and 2.0% depending on particle size and season) do exhibit timescales larger than this. The accommodation coefficient of this minority of particles is estimated to be approximately in the range of 1 to 4 ¿ 10-5, which is in the lowest range of observed values for laboratory aerosols. The observations show a strong dependence of the occurrence of such particles on season and time of day. It is hypothesized that these measurements suggest that an even larger concentration of particles exhibiting accommodation coefficients in the range of 10-5 to 10-4 may exist. The existence of such particles would be important to the indirect effect of aerosols on climate.