This work focuses on interstitial solid solutions of hydrogen in the face centered cubic (fcc) host lattice of palladium and nickel, using a first-principles based approach. Cluster Variation Method (CVM) and Monte Carlo simulation algorithms were especially designed, allowing a coupled use of both techniques, to study hydrogenvacancy interactions inside a fcc metallic host lattice. First-principles calculations provided the H-Vac interaction energies by structure inversion method. The phase diagrams and thermodynamic properties were computed using only theoretical inputs. The mechanisms leading to the formation of the miscibility gaps observed for both Pd-H and Ni-H systems and the hydrogen ordering on palladium interstitial lattice were reproduced without any empirical term.