- The main goal of this communication is to describe an experimental detection of a new physical phenomenon of turbulent thermal diffusion which was predicted theoretically in Phys. Rev. Lett. 76, 224 (1996). In a turbulent fluid flow with a nonzero mean temperature gradient there appears an additional mean flux of particles in the direction opposite to the mean temperature gradient (the phenomenon of turbulent thermal diffusion). For large Reynolds and Peclet numbers the turbulent thermal diffusion is much stronger than the molecular thermal diffusion. The phenomenon of turbulent thermal diffusion has been detected experimentally in oscillating grids turbulence with an imposed linear stable temperature gradient (up to 120 K/m) in air flow. We used Particle Image Velocimetry to determine the turbulent velocity field and an Image Processing Technique to determine the spatial distribution of particles. Analysis of the intensity of laser light Mie scattering by micron particles showed that they are accumulated in the vicinity of the minimum of the mean temperature of the surrounding fluid due to the effect of turbulent thermal diffusion. The effect of turbulent thermal diffusion is important in atmospheric phenomena (e.g., atmospheric aerosols, cloud formation and smog formation) and industrial turbulent flows (e.g., internal combustion engines). In particular, this effect results in the formation of large-scale inhomogeneities in spatial distribution of aerosol particles in the vicinity of the temperature inversion.