High-pressure mass diffusion and Soret coefficients of the equimassic 1,2,3,4-tetrahydronaphthalene and n-dodecane binary mixture are obtained from dynamic light scattering analysis of concentration non-equilibrium fluctuations at the steady state of Soret-driven separation. A high-pressure shadowgraph set-up has been developed to investigate thermodiffusion in free medium from atmospheric pressure up to 20MPa. Results at atmospheric pressure show excellent agreement with benchmark values. High-pressure results for the mass diffusion coefficient confirm theoretical predictions by Leffler-Cullinan relation. Further calculation of the thermodiffusion coefficient allows also comparison with previous experimental results with, again, very good agreement. Graphical abstract: [Figure not available: see fulltext.