A virial equation of state for rubidium with coefficients through the fifth virial is predicted from experimental PVT data of potassium and cesium by application of the thermodynamic similarity principle. Exact thermodynamic relationships involving virial coefficients and other selected literature properties are used to generate saturation and superheat properties of rubidium vapor. Values of enthalpy, entropy, specific heat, and specific volume are tabulated for some 200 selected states in the temperature range from 1300°to 2400°F. and in the pressure range from 0.2 to 28 atm.