This work reports new experimental data for the density and viscosity of n-decane + CO₂ binary mixtures. A simultaneous density and viscosity measurement apparatus was redesigned using a vibrating–wire viscometer coupled with a vibrating–tube densimeter. The measurements for the pure component (n-decane) and three different binary mixtures (n-decane + CO₂), with mole fractions of n-decane of (0.4026, 0.2225, 0.1262), were carried out at temperatures ranging from (303.15 to 373.15) K and pressures up to 80 MPa. The expanded relative uncertainties (k = 2, level of confidence = 0.95) were estimated to be 0.2% for density and 3.0% for viscosity over the entire measurement range, respectively. Densities were correlated as a function of temperature and pressure through a modified Tait–type equation with an average absolute percentage deviation less than 0.05%. To model the viscosity of the binary mixtures as a function of temperature, density, and composition, the extended hard–sphere method combined with simple quadratic mixing rules were used to describe our experimental data. Good agreement is found between experiments and correlations, which attests to the predictive capabilities of semi–empirical equations. In addition, Monte Carlo simulations in the isobaric-isothermal ensemble with the Transferable Potential for Phase Equilibria–United Atom (TraPPE–UA) force field were carried out to characterize the microscopic structure of the studied systems.