The thermodynamic behaviour of some liquids can be described by a simple isotropic potential of intermolecular interaction. In the first place among such liquids are technically important liquefied gases — the working medium of refrigeration, cryogenic and power installations. A model of a liquid condition based on the behaviour analysis of the liquid that is equilibrium with steam as well as overheated and overcooled to the temperature of 0 K has been suggested. The obtained equation of the condition that included the curve of the ideal gas and a so-called cold isotherm that corresponded to the temperature of 0 K was characterized by simplicity and sufficient accuracy for the thermodynamic provision of calculations.