The crystal structure and magnetothermal properties of the potential magnetic refrigerant materials,Tb5(SixGe4−x)alloys wherex⩾2,have been studied. The crystal structure of theTb5(SixGe4−x)alloys varies from the orthorhombicGd5Si4-type to the monoclinicGd5(Si2Ge2)-type as silicon concentration decreases fromx=4tox=2.The dc magnetic susceptibility and magnetization data indicate that a paramagnetic to ferromagnetic phase transition occurs in these alloys. The heat capacity data measured in magnetic fields varying from 0 to 100 kOe show that a second order(x=4,3)or a first order phase transition(x=2)occurs in the alloys. The magnetic entropy change and the adiabatic temperature change calculated from the heat capacity data indicate that theTb5(SixGe4−x)alloys are promising materials for magnetic refrigeration from about 100 to 250 K when their chemical composition varies fromx=2tox=4.©2002 American Institute of Physics.