The saturation vapor of an alkali metal is treated as a monatomic particle real gas whose volumetric behavior is described by a truncated form of virial equation. The condensed state-vapor equilibrium constant, equal to the saturation fugacity for Li, Na, K, and Cs, is determined through evaluation of the second virial coefficient's temperature dependence. The third and the second law values of standard enthalpy of vaporization at the reference temperature 298 K for the same metals are determined. The applied procedure for evaluation of the second virial coefficient's temperature dependence produced small deviations for the resulting second law values and gave mean values in good agreement with literature data, especially with those of the JANAF Tables.