Densities of sulfur dioxide (SO2) have been measured with a computer-controlled high-temperature, high-pressure vibrating-tube densimeter system (DMA-HDT) in the sub- and supercritical states. The densities were measured at temperatures from 273 to 523 K and pressures up to 35 MPa (363 overall data points), for which a density range between 194 and 1485 kg/m(3) was covered. The uncertainty in density measurement was estimated to be better than 0.1% in the liquid and compressed supercritical states. Near the critical temperature and pressure, the uncertainty increases to a maximum of 1%. Using the measured densities and additional thermodynamic data from the literature, a 12-parameter equation explicit in Helmholtz energy was developed for SO2. The equation of state (EoS) is valid from 197.7 to 523 K and for pressures up to 35 MPa. The uncertainty in density of the EoS ranges from 0.1% at low temperatures in the liquid and vapor to 0.5% at the highest temperatures. The uncertainty in heat capacities is 2%, and the uncertainty in vapor pressure is 0.4% at temperatures above 270 K. In the critical region, the uncertainties are higher for all properties except vapor pressure. (C) 2003 Elsevier Science B.V All rights reserved.