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Densities of sulfur hexafluoride (SF6) and dinitrogen monoxide (N2O) have been measured with a fully computer-controlled high-temperature high-pressure vibrating tube densimeter system in the sub- and supercritical states. The uncertainty in density measurement was estimated to be between ±0.2 and ±0.3 kg·m−3 depending on the temperature. With respect to accuracy, reliability, suitability, and time consumption, this system has significant advantages for measuring PρT properties in the compressed liquid and supercritical states. The densities were measured for temperatures from 273 to 623 K and at pressures up to 30 MPa for SF6 (442 data points) and from 273 to 473 K and up to 40 MPa for N2O (251 data points), which encompassed density ranges between 142.9 and 1778.5 kg·m−3 for SF6 and between 124.4 and 1051.1 kg·m−3 for N2O. Furthermore, the liquid densities of SF6 and N2O were correlated with a new three-dimensional density correlation system (TRIDEN) and the complete set of PρT data in the sub- and supercritical states were correlated with a virial-type equation of state. For checking the accuracy and suitability of the vibrating tube densimeter system, the experimental densities of SF6 were compared with published data and with the results of a reference equation of state.