The room temperature crystal structures of Er5SixGe4−x alloys change systematically with the concentration of Ge from the orthorhombic Gd5Si4-type when x=4, to the monoclinic Gd5Si2Ge2 type when 3.5⩽x⩽3.9 and to the orthorhombic Sm5Ge4 type forx⩽3. The Curie-Weiss behavior of Er5SixGe4−x materials is consistent with the Er3+ state. The compounds order magnetically below 30 K, apparently adopting complex noncollinear magnetic structures with magnetization not reaching saturation in 50 kOe magnetic fields. In Er5Si4, the structural-only transformation from the monoclinic Gd5Si2Ge2-type to the orthorhombic Gd5Si4-type phase occurs around 218 K on heating. Intriguingly, the temperature of this polymorphic transformation is weakly dependent on magnetic fields as low as 40 kOe (dT∕dH=−0.058 K∕kOe) when the material is in the paramagnetic state nearly 200 K above its spontaneous magnetic ordering temperature. It appears that a magnetostructural transition may be induced in the 5:4 erbium silicide at ∼18 K and above by 75 kOe and higher magnetic fields. Only Er5Si4 but none of the other studied Er5SixGe4−x alloys exhibit magnetic field induced transformations, which are quite common in the closely related Gd5SixGe4−x system. The magnetocaloric effects of the Er5SixGe4−x alloys are moderate.