© 2020 SETAC Fertilizers containing phosphate (PO43−) are commonly used within the agricultural industry and are known to increase the bioavailability and mobility of metalloids like arsenic (As). This may increase plant uptake of As and hence pose a risk to human health. Arsenic and antimony (Sb) often co-occur in contaminated soils; however, little is known about the interactions between As and Sb with PO43− on their bioavailability, accumulation, and toxicity in plants. The present study investigated individual and combined As and Sb–contaminated soils across 2 soil PO43− concentrations using a commonly consumed leafy vegetable, choy sum (Brassica chinensis var. parachinensis). Increased soil PO43− had no clear influence on the bioavailability of As or Sb (derived from a sequential extraction procedure). At high PO43− concentration, B. chinensis accumulated higher amounts of As in the shoots and roots in both individual and co-contaminated soil, whereas Sb accumulation increased only when Sb was the only contaminant. When As was the only contaminant, the translocation of As from roots to shoots decreased as soil PO43− increased. Increased soil PO43− had no influence on Sb translocation from root to shoot. Although As was toxic (impaired growth) at low PO43− soil concentration, no toxicity was observed in the high-PO43− soil. No toxicity was observed for Sb in either low- or high-PO43− soils. Increased soil PO43− concentration ameliorated or masked As toxicity to plant growth and led to higher As concentration in the plant's edible parts. The addition of high soil PO43− concentrations ameliorated or masked As toxicity to plant growth in both individually and As + Sb co-contaminated soil; however, the plant's edible parts accumulated higher As and Sb concentrations. Environ Toxicol Chem 2020;00:1–11. © 2020 SETAC.