Gd5Si2Ge2 parent compounds were alloyed with Mn in order to understand the underlying relation between the structural phases and the magnetic behavior of the pseudo ternary compounds formed. The alloying mechanism in Gd5Si2Ge2 causes simultaneous substitution of the nonmagnetic Si and Ge atoms from the (Si + Ge) sublattice in equal amounts. No subsequent heat treatment was made on alloyed compounds. X-ray powder diffraction, magnetization versus temperature and isothermal magnetization measurements were carried out. X-ray diffraction patterns were used to qualitatively determine the existence of different structural phases in the alloys. It was observed that the starting, as-melted alloy with z=0 has Gd5Si4-tYPe orthorhombic structure at room temperature with traces of 1: 1 stoichiometry phase which transforms totally to a Gd5Si2Ge2-tYPe monoclinic phase when heat treated. Similarly, increase in the Mn content leads to an increase in the monoclinic phase content of the originally orthorhombic compounds. Curie temperatures were determined from M(T) measurements and the magnetocaloric characterization was made using M(H) measurements by plotting the magnetic entropy change values against temperature. No giant magnetocaloric effect was observed for non heat treated samples. (c) 2005 Elsevier B.V All rights reserved.