A thermodynamic model of a partially open two-phase system initiated by Schwarz and Khachaturyan is extended for the hydrogen storage investigation. A particular attention is paid to the modeling of the evolution of hydrogen content absorbed in the solid solution as a function of the pressure of hydrogen gas (for isothermal external conditions), e.g. PCI curves. The extension concerns the description of anhysteretic PCI curves with plateau or slope. Finally, the experimental data including anhysteretic behavior and also hysteretic one are fairly described by our modeling.