An equation of state has been developed to represent the thermodynamic properties of all liquid, vapor, and supercritical states of n-dodecane. Experimental data used to develop the equation included pressure-density-temperature state points, vapor pressures, isobaric and saturation heat capacities, heats of vaporization, and speeds of sound. The uncertainties of properties calculated using the equation are as follows: 0.2% in density at pressures up to 200 MPa, and 0.5% at higher pressures up to 500 MPa; 1% in heat capacity; 0.5% in the speed of sound; and 0.2% in vapor pressure. Deviations of calculated properties from available experimental data in the critical region are higher for all properties except vapor pressure.