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New seawater density measurements were made as a function of temperature <i>T</i>=(273.15 to 363.15) K and salinity (5 to 70 g kg<sup>−1</sup>). The measurements (<i>N</i>=230) from <i>T</i>=273.15 to 313.15 K and Practical Salinity (<i>S</i>) from 0 to 40 were found to be in good agreement (σ=0.0036 kg m<sup>−3</sup>) with the equation of state of seawater (Millero and Poisson, 1981) made on samples with a known chlorinity (Cl). These results indicate that the Practical Salinities (<i>S</i>) are in agreement to within ±0.003 kg m<sup>−3</sup> with the values calculated from the Chlorinity, <i>S</i><sub>Cl</sub>=1.80655 Cl. The measurements from 298.15 to 363.15 were used to extend the equation of state to high temperatures and salinities. All the densities were made relative to pure water (ρ−ρ<sup>0</sup> where ρ<sup>0</sup> is the density for pure water) were fitted to equations of the form <br><br> (ρ−ρ<sup>0</sup>)/(kg m<sup>−3</sup>)=<i>A S<sub>A</sub>+B S<sub>A</sub><sup>1.5</sup>+C S<sub>A</sub><sup>2</sup></i> <br><br> where <i>A</i>, <i>B</i>, and <i>C</i> are functions of temperature and <i>S<sub>A</sub></i> (g kg<sup>−1</sup>) is the absolute salinity, <i>S<sub>A</sub></i>=(35.16504/35) <i>S</i> g kg<sup>−1</sup>. The fitted results from <i>S<sub>A</sub></i>=0 to 50 g kg<sup>−1</sup> and <i>T</i>=273.15 to 313.15 K (<i>N</i>=242) gave standard errors of 0.0037 kg m<sup>−3</sup>. The fitted results from 298.15 to 363.15 K (<i>N</i>=280) gave standard errors of 0.0063 kg m<sup>−3</sup> and all the results (<i>N</i>=522) from 273.15 to 363.15 K gave standard errors of 0.0063 kg m<sup>−3</sup>. The earlier density measurements (Millero et al., 1976b; Poisson et al.,1980) used to determine the equation of state of seawater (Millero and Poisson, 1981) were combined to derive equations that are valid from 273.15 to 313.15 K and 273.15 to 363.15 K. The standard errors of these fits are respectively, 0.0038 kg m<sup>−3</sup> (<i>N</i>=713) and 0.0063 kg m<sup>−3</sup> (<i>N</i>=962). These new measurements expand the equation of state of seawater to a wider range of temperature (273.15 to 363.15) K and absolute salinity (0 to 70 kg m<sup>−3</sup>.