This paper presents a method to coat nanometer-sized aerosol particles with thin films of organic materials. The method consists of continuous mixing of the aerosol with heated air saturated with organic vapor. During the process, the organic vapor condenses on the surface of the aerosol particles (seeds) forming a coating film on them. The coating film thickness, estimated through measurement of aerosol particle size distribution by a differential mobility analyzer before and after coating, turned out to be easily controlled by adjusting two parameters: the temperature of vapor-saturated air and the mixing ratio of vapour-saturated air flow rate to aerosol flow rate. For a given size of the seed particles, there exists a critical value of the coating film thickness above which the film does not allow water permeation towards the core seed. The critical thickness has the same order of magnitude as the seed diameter and is proportional to it. A simple theoretical model of the coating film growth process, yielding results in agreement with the experimental trends, is presented.