Recent measurements of critical supersaturations for the vapor phase homogeneous nucleation of several substances using a diffusion cloud chamber technique exhibit a dependence on the pressure of the carrier gas used in the experiments. A model of droplet growth and motion in a diffusion cloud chamber, combined with the density and temperature profiles of the chamber is presented to explain the pressure dependent results. The model demonstrates that at higher carrier gas pressures the growth of the droplets is retarded and the optical scattering signal from the particles is reduced. It is concluded that the observed effect may not result from a pressure dependence of the nucleation rate, but from a pressure dependence of the droplet growth and motion.