A pseudopotential method has been considered to study the enthalpy and compressibility of liquid alkali metals (Na, K, Rb and Cs) as a function of temperature. The diameters of the hard sphere reference system were determined by minimising the Helmholtz free energy to ensure thermodynamic stability. The enthalpy, which depends weakly on temperature, has been successfully explained. The investigation suggests that the structure dependent part of energy is sufficient to explain the compressibility of liquid alkali metals near and above the melting temperatures.