This study focuses on investigating the phase transitions in two materials, Cu2ZnSnS4 (CZTS) and CuZnGeS4 (CZGS), which are important for understanding their structural and functional properties. The temperature and pressure-induced tetragonal-orthorhombic phase transitions in these materials are analyzed using density functional theory (DFT) and the quasi-harmonic Debye model. The research aims to examine the changes in the material's structure and the associated thermodynamic properties during these phase transitions. The results reveal that both compounds exhibit a negative value of ΔHmix, indicating the release of energy during the mixing process, which suggests an exothermic nature. Our DFT calculations at zero temperature and pressure, demonstrate that the Stannite structure represents the ground state configuration of the Cu2ZnSnS4 system (with xGe = 0%), compared to the Wurtzite-Stannite structure. The calculations also show that the difference in enthalpies of formation (∆H) between the Stannite and Wurtzite-Stannite phases for CZTS is estimated to be 8.884meV per atom. Regarding Cu2ZnGeS4, the Wurtzite-Stannite structure is found to be the most stable, closely followed by the Stannite structure, with enthalpies of formation of -4,833eV.atom-1 and -4,804eV.atom-1, respectively. Notably, there are no definitive reports on enthalpy studies for the Cu2ZnGeS4 system in the existing literature. Furthermore, the DFT studies indicate that the energy difference between the Stannite and Wurtzite-Stannite phases in CZTS is smaller than that in CZGS, amounting to 29.195eV per atom. The temperature has minimal influence on the thermodynamic Gibbs energy of these quinary alloys. This suggests that the ability of these alloys to undergo changes in their crystal structure is not strongly dependent on thermal energy. While maintaining a constant temperature at 0K, it has been observed that the effect of pressure on the Gibbs energy for both compounds is relatively small. However, it is worth noting that at higher pressures (>45 GPa), a phase transition from Stannite (St) to Wurtzite-Stannite (WSt) occurs for CZTS, and from WSt to St for CZGS. Understanding the behavior of these materials under different conditions can contribute to the development of improved performance and stability of devices based on CZTS and CZGS.