Abstract In order to tune the magnetic properties of the cleavable high-Curie temperature ferromagnet Fe 5 − x GeTe₂, the effect of increasing the electron count through arsenic substitution has been investigated. Small additions of arsenic (2.5% and 5%) seemingly enhance ferromagnetic order in polycrystalline samples by quenching fluctuations on one of the three magnetic sublattices, whereas larger As concentrations decrease the ferromagnetic Curie temperature (T _C) and saturation magnetization. This work also describes the growth and characterization of Fe_4.8AsTe₂ single crystals that are structurally analogous to Fe 5 − x GeTe₂ but with some phase stability complications. Magnetization measurements reveal dominant antiferromagnetic behavior in Fe_4.8AsTe₂ with a Néel temperature of T N ≈ 42 K. A field-induced spin-flop below T _N results in a switch from negative to positive magnetoresistance, with significant hysteresis causing butterfly-shaped resistance loops. In addition to reporting the properties of Fe_4.8AsTe₂, this work shows the importance of manipulating the individual magnetic sublattices in Fe 5 − x GeTe₂ and motivates further efforts to control the magnetic properties in related materials by fine tuning of the Fermi energy or crystal chemistry.