To examine the effect of thermal diffusion on thermal CVD, compositional separation by thermal-diffusion effect in a locally heated rectangular flow channel was investigated experimentally and theoretically. Experiments were conducted for two binary gas mixtures (He-SF6 and Ne-SF6). The local concentration of SF6 was measured by a QMS by using a fused quartz capillary tube (75 mum I. D. and 900 mm in length) inserted into the flow channel to sample the mixtures. A series of three-dimensional numerical simulations of thermal diffusion in complex flow structures by means of the control volume method was conducted for the flow system. The numerical results showed reasonable agreement with the experimental results for these gas mixtures, for which an accurate value of thermal diffusion factor is known.