Comprehensive (p, rho, T) measurements on pure ethylene (83 values), pure ethane (168 values), and pure sulphur hexafluoride (220 values) have been carried out in the homogeneous gas and liquid phases for temperatures from 235 K to 520 K at pressures up to 30 MPa. The measurements were performed by means of an accurate single-sinker densimeter based on the Archimedes' buoyancy principle. The total uncertainty of the measurements in density p was estimated to be +/-(0.02 to 0.025)%. The measurements have been compared with results of other experimentalists and with values calculated from current equations of state.