We study the electronic structure and magnetic properties of Gd5(Si1-xGex)4 system of alloys using band structure methods. The total energy, atomic magnetic moments are compared for the monoclinic and orthorhombic crystallographic structures. We confirmed that the orthorhombic structure has lower energy compared to the monoclinic one. The total energy calculations for the transformation from orthorhombic to monoclinic structure revealed no energy barrier. The addition of spin-orbit coupling does not change this result. The exchange coupling calculations show that effective Heisenberg model parameters do not exceed 100 K and decrease with transition from orthorhombic to monoclinic structure. We found that the total energy is sensitive to the specific ordering of Ge and Si atoms. This result agrees with experimental data of occupation numbers of T and T′ positions and may be related to the anomalies observed experimentally. © 2002 American Institute of Physics.