The room temperature crystal structures of Er₅Si_xGe_4−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 Er₅Si_xGe_4−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 Er₅Si_xGe_4−x alloys exhibit magnetic field induced transformations, which are quite common in the closely related Gd₅Si_xGe_4−x system. The magnetocaloric effects of the Er₅Si_xGe_4−x alloys are moderate.