The integration of III–V semiconductors into silicon technology remains one of the key challenges for the development of efficient silicon-based optoelectronic devices. In this work, the thermodynamic stability and phase stability of a continuous solid solution (Si₂)₁₋ₓ(GaP)ₓ formed via molecular substitution, in which GaP structural units replace pairs of Si–Si atoms in the crystalline silicon lattice, are investigated. Formation energies calculated using density functional theory (DFT) confirm the possibility of experimental realization of stable (Si₂)₁₋ₓ(GaP)ₓ solid solutions.