New optimized experimental setup based on pendant drop tensiometry was developed, and a mathematical model designed to fit the experimental data was used to determine the diffusion coefficients of binary systems at elevated pressures and temperatures. For the first time, the diffusion coefficients of propylene glycol in super-critical CO2 were determined at temperatures of 398.2 and 423.2 K and at pressures from 5 MPa, up to 17.5 MPa. Experimental procedure was validated by a comparison of the experimental data for the mixture water-CO2 with data from the literature. For this purpose, the diffusivity of water in CO2 was measured at 298.15 K in the pressure range from 5 MPa, up to 60 MPa. Accuracy of results was certified by measuring more than 1000 points of equivalent drop diameter. Droplet geometry was examined by using a precise computer algorithm that fits Young-Laplace equation to the axisymmetric shape of a drop.