We present a new type of thermogravitational (TG) column, a so-called TG microcolumn with transparent windows and,a very small sample volume of less than 50 mu L. The TG microcolumn has a planar geometry with a thickness of 0.523 +/- 0.004 mm, a height of 30 mm, and a width of 3 mm. The concentration difference between two points at different heights is Measured with an interferometer using active phase control. From the concentration difference we can determine the thermal diffusion coefficient, D-T, using the. refractive index variation with concentration, which has to be determined independently. We studied the: three binary mixtures of dodecane, isobutylbenzene, and 1,2,3,4-tetrahydronaphthalene with a concentration of 50 wt % rat a temperature.., of 298 K. The thermal diffusion coefficients agree within a few percent with the. proposed benchmark values In addition we investigated also the binary mixture toluene/n-hexane and compare the results.: with literature values: For the investigated.: mixtures. the typical measurement times were between. 30 min and 2 h with an applied temperature difference Of Delta T = 6 K.