Comprehensive (rho, rho, T) measurements on pure sulphur hexafluoride have been carried out in the single-phase region (609 values) and along the entire (vapour + liquid) coexistence curve (95 values). The results reported in this first paper cover the homogeneous gas and liquid region in the temperature range from 225 K to 340 K at pressures up to 12 MPa. The total uncertainty of the measurements in density is less than or equal to+/- (0.015 to 0.023) per cent with the exception of the critical region for temperatures from 316 K to 324 K (T-c = 318.723 K) at densities between 450 kg (.) m(-3) and 1000 kg (.) m(-3) (rho(c) = 742.26 kg (.) m(-3)), where the total uncertainty in pressure is less than or equal to+/- (0.007 to 0.011) per cent. Values for the second and third virial coefficients have been determined from the measurements in the homogeneous gas region of sulphur hexafluoride. The experimental results have been compared with the results of other workers.