A new equation of state for the thermodynamic properties of natural gases, similar gases, and other mixtures, the GERG-2008 equation of state, is presented in this work. This equation is an expanded version of the GERG-2004 equation. GERG-2008 is explicit in the Helmholtz free energy as a function of density, temperature, and composition. The equation is based on 21 natural gas components: methane, nitrogen, carbon dioxide, ethane, propane, n-butane, isobutane, n-pentane, isopentane, n-hexane, n-heptane, n-octane, n-nonane, n-decane, hydrogen, oxygen, carbon monoxide, water, hydrogen sulfide, helium, and argon. Over the entire composition range, GERG-2008 covers the gas phase, liquid phase, supercritical region, and vapor-liquid equilibrium states for mixtures of these components. The normal range of validity of GERG-2008 includes temperatures from (90 to 450) K and pressures up to 35 MPa where the most accurate experimental data of the thermal and caloric properties are represented to within their accuracy. The extended validity range reaches From (60 to 700) K and up to 70 MPa. The given numerical information (including all of the sophisticated derivatives) enables the use of GERG-2008 for all of the various technical applications. Examples are processing, transportation through pipeline:: or by shipping, storage and liquefaction of natural gas, and processes to separate gas components. Comparisons with other equations of state, for example, AGA8-DC92 and Peng-Robinson equation (P-R), are also presented. GERG-2008 will be adopted as an ISO Standard (ISO 20765-2/3) for natural gases.