This work reviews the available data on thermodynamic properties of argon and presents a new equation of state in the form of a fundamental equation explicit in the Helmholtz energy. The functional form of the residual part of the Helmholtz energy was developed by using state-of-the-art linear optimization strategies and a new nonlinear regression analysis. The new equation of state contains 41 coefficients, which were fitted to selected data of the following properties: ͑a͒ thermal properties of the single phase (pT) and ͑b͒ of the liquid-vapor saturation curve ͑p s , Ј, Љ͒ including the Maxwell criterion, ͑c͒ speed of sound w, isochoric heat capacity c v , second and third thermal virial coefficients B and C and second acoustic virial coefficient ␤ a. For the density, the estimated uncertainty of the new equation of state is less than Ϯ0.02% for pressures up to 12 MPa and temperatures up to 340 K with the exception of the critical region and less than Ϯ0.03% for pressures up to 30 MPa and temperatures between 235 and 520 K. In the region with densities up to half the critical density and for temperatures between 90 and 450 K the estimated uncertainty of calculated speeds of sound is in general less than Ϯ0.02%. The new formulation shows reasonable extrapolation behavior up to very high pressures and temperatures. Independent equations for the vapor pressure, for the pressure on the sublimation and melting curve and for the saturated liquid and saturated vapor densities are also included. Tables for the thermodynamic properties of argon from 84 to 700 K for pressures up to 1000 MPa are given.