Data on the thermal forces acting on aerosol particles in a temperature gradient are relatively few. Measurements are reported here on droplets of paraffin oil and castor oil suspended in air, showing the effect of temperature gradient, particle size, and particle material. The thermal force is directly proportional to the temperature gradient prevailing in the gas and to the particle diameter. It is an inverse function of the thermal conductivity of the aerosol material. The agreement between the experimental data and the theory of P. S. Epstein [Z. Physik54, 537 (1929)] is satisfactory, a fortunate result in view of the approximations to the actual physical situation made in the derivation of the theoretical equation. The velocity of migration of aerosol particles in a temperature gradient can be estimated from Epstein's formula.