Using suggested by G.Shubin base equation and equations for wood porosity, partial pressure of dry air and water vapor, wood density, etc., a mathematical description of the specific mass capacity of the wood in relation to the steam-air mixture during its molar transfer process cpm in hygroscopic diapason has been made. The influence of temperature T, pressure P, moisture content U on cpva is taken into consideration. The mathematical description of cpva can be used for the computation of the different processes of hydro-heat treatment of wood with aim of optimization of their technology and automatic control.