The first experimental evidence that noncolloidal particles exhibit a thermophoretic migration when placed in a temperature gradient is provided by observing the retention of micron-sized porous spherical particles, used as packing in liquid chromatography, in a thermal field-flow fractionation instrument. To eliminate the perturbing gravitational effects, the channel flow is set vertically and the temperature gradient horizontally. A method of determination of the thermophoretic mobility is developed. It relies on the dependence of the amount of retained particles on the duration of the initial stop-flow step immediately after sample introduction. This method may be implemented for the determination of particle mobility with any kind of applied force field. The value of the thermophoretic mobility is found to depend on the chemical nature of the particle surface, but not significantly on the nature of the pure suspending liquid. For silica particles, it is slightly larger in water than in acetonitrile. In this latter liquid, the thermophoretic mobility of pure silica particles is about 3 times larger than that of octadecyl-bonded silica particles. This demonstrates that, for micron-sized particles, thermophoresis is governed by surface effects and may be the basis of a process for characterizing the surface properties of particulate materials.