Effect of Dy substitution at the Gd-site on the magnetic and electrical transport properties of the giant magnetocaloric effect material, Gd5Si2Ge2, has been explored. At room temperature, DyxGd5-xSi2Ge2 (x≤3.5) compounds have the monoclinic Gd5Si2Ge2-type crystal structure (Space group P21/a), while those with x>3.5, have orthorhombic Sm5Ge4-type structure (Space group Pnma). The Dy substitution contracts the unit cell and also reduces the monoclinic distortion. The compound Gd5Si2Ge2 orders ferromagnetically at 276 K; with increasing Dy substitution, the magnetic transition temperature decreases (TN=56 K for Dy5Si2Ge2) and the nature of magnetic coupling becomes antiferromagnetic in Dy-rich end. The electrical resistivity shows a marked drop near TC/TN. The thermoelectric power undergoes a slope change in the vicinity of magnetic transition and the signature of phonon and magnon drag effects is observed at low temperatures. Thus progressive Dy substitution in Gd5Si2Ge2 leads to a class of materials with interesting magnetic ground states, with magnetic ordering temperatures spanning from 56 to 276 K. © 2006 Elsevier B.V. All rights reserved.