Gas permeabilities across poly(chloro-p-xylylene) (parylene C) films are measured with different thicknesses of 20.2, 10.0, 8.9, 4.6, 3.4, and 1.0 μm. Measurements were carried out below 1 atm and between 10 and 80°C, which are under the glass transition temperature. The temperature and pressure dependencies of the permeability and the apparent diffusion coefficients were measured. If the membrane thickness is larger than 8 μ, the gas-transport mechanism is solution–diffusion, which implies that it is pinhole-free, because the pressure dependency of the permeability cannot be found and the apparent activation energy of permeation and diffusion are observed. If the membrane thickness is less than 8 μ, the gas transport mechanism is pore flow combined with solution–diffusion flow because gas may penetrate both the porous area and the polymer matrix. The thinner the membrane, the higher is the permeability coefficient, since the diameter and number of pores increase with decrease of the membrane thickness. The gas permeability coefficient has different values at the same pressure or temperature. As this film is in the glassy state, it should be explained using the average ordering parameter (ξ), which is a function of temperature, pressure, gas concentration, and time. © 1994 John Wiley & Sons, Inc.