A giant magnetocaloric effect(&Dgr;Smag)has been discovered in theGd5(SixGe1−x)4pseudobinary alloys, wherex⩽0.5.For the temperature range between∼50and∼280 Kit exceeds the reversible (with respect to alternating magnetic field)&Dgr;Smagfor any known magnetic refrigerant material at the corresponding Curie temperature by a factor of 2–10. The two most striking features of this alloy system are: (1) the first order phase transformation, which brings about the large&Dgr;SmaginGd5(SixGe1−x)4,is reversible with respect to alternating magnetic field, i.e., the giant magnetocaloric effect can be utilized in an active magnetic regenerator magnetic refrigerator; and (2) the ordering temperature is tunable from∼30to∼276 Kby adjusting the Si:Ge ratio without losing the giant magnetic entropy change. ©1997 American Institute of Physics.