Discoloration of bermudagrass often results from application of MSMA herbicide used to control southern crabgrass and other weeds. However, when products containing iron sulfate (FeSO4) are tank-mixed with MSMA, this discoloration is reduced. Experiments investigated the effect of tank-mixing organic arsenical herbicides with FeSO4 or a chelated iron source (Sprint 330) in terms of southern crabgrass control and injury to bermudagrass. Tank-mixing MSMA with FeSO4 reduced bermudagrass injury. However, southern crabgrass control was also reduced by at least 50% with the addition of ≥0.38 kg Fe2+ ha−1. Neither antagonism nor safening of bermudagrass was observed when the chelated Fe2+ source was used. Applying FeSO4 as a separate treatment 1 to 4 d before or after MSMA application did not reduce visual burmudagrass injury 1 wk after treatment. Solution pH and FeSO4 concentration controlled the extent of complexation and level of antagonism observed in the field; inorganic Fe2+ reacted with MSMA to form a complex having reduced herbicidal activity. Potentiometric and spectrophotometric investigations found that methylarsonate, the parent acid of MSMA and other organic arsenical herbicides, reacts with inorganic Fe2+ to form a stable 1:1 Fe2+-methylarsonic acid chelate having two points of metal coordination and a stability constant log10 (β) = 2.77 ± 0.04. Tank-mixing MSMA with FeSO4 to protect against bermudagrass injury negates the benefit of applying the herbicide for weed control, and therefore is not a recommendable practice for turf managers.