The objective is the measurement of the thermophoretic forces acting on single particles suspended in an electrodynamic balance, and the evaluation of existing theories related to this experiment. The focus is on relatively large particles and high thermal conductivities, for which negative thermophoretic effects have been suggested. Single particles will be suspended in an electrodynamic balance and will be sized by use of their Mie resonance and angular spectra. Thermal gradients from 0 to 75 K/cm and mean temperatures from 250 to 400 K will be studied. The research is of fundamental nature, and has relevance to several industrial processes, including manufacture of optical fibers, clean room technology, particle erosion and deposition processes, as well as pollution and combustion.