THE velocity of thermophoresis v of small aerosol particles of radius R, much smaller than the free path λ of the surrounding gas molecules, has been calculated by us1 in two ways: (1) as a limiting case of thermodiffusion; (2) by finding the momentum imparted to the particles by impact of molecules the velocity distribution of which is not disturbed by its presence, using the method of Chapman et al.2. The formula obtained for the case of diffusional molecular scattering: where T is the Kelvin temperature, η and ρ the viscosity and density of the gas, reveals that v is independent of the thermal conductivity of the particles, in contrast to the case where R ≫ λ.