The results of modelling of the equilibrium composition and thermodynamic characteristics of the Nb - Та - Mo — W system with equiatomic initial content of components in argon atmosphere in the pressure range 10ч - 108 Pa at temperatures range 300 - 10000 К are presented. As the software the TERRA package was used. The modelling system consists of a condensed phase (ideal Nb - Та - Mo - W solution) and a gas phase above it. The content of components is determined by the equilibrium state of the whole system. The following components were considered in the modelling: gaseous: Ar, Nb, Та, Mo, W, Ar+, Nb+. Ta+, Mo+, W+, electron gas e", and condensed: Nb. Та, Mo, and W. Temperature dependences of the content of components of condensed and gas phases formed during heating of this alloy, as well as thermodynamic characteristics (enthalpy, entropy and Gibbs energy) of the modelled system at different pressures were calculated. The regularities of behaviour of components are revealed. It is shown that temperature dependences of thermodynamic characteristics are not monolonic, but have breaks due to phase transformations that occur at equilibrium heating. The temperature at which evaporation of the melt occurs increases with pressure.