This paper describes the measurement of the Soret coefficient of a hydrocarbon binary liquid mixture at different orientations to the gravity vector. The working fluid in the experiment was composed of 1,2,3,4-tetrahydronaphtalene (THN) and dodecane (C-12) at mass fractions of 0.50/0.50 and a temperature of 298.15 K. The experimental technique used in this study was an optical digital interferometry method using a Mach-Zehnder Interferometer. A fast Fourier transform method was used for image processing. The experimental approach was supplemented with a numerical solution using the finite volume method. Experimental and numerical results both confirmed that cell inclination during the thermodiffusion process had a tremendous effect on the measurement of the Soret coefficient.