AbstractThis paper discusses the factors controlling the rate of loss of plasticiser from polyvinyl chloride compositions to a surrounding gas phase. These are (a) the rate of diffusion in the solid phase or (b) that in the gas phase; for any set of conditions one or other of these will usually be predominant. The rate of desorption from the solid to the gas phase is not considered likely to be a controlling factor in practical circumstances. When gas phase diffusion is rate‐controlling, the loss rate will depend greatly on the degree of turbulence in the gas phase. Since the diffusion contents of ordinary plasticisers calculated from Gilliland's equation1 do not differ as much as the vapour pressures of the plasticisers, the rate of loss will be roughly proportional to the vapour pressure, at different temperatures or for different plasticisers. When solid phase diffusion is rate‐controlling the loss rate, independent of air velocity, will depend on the dimensions of the sample, particularly on its thickn