Environmental context. Arsenic (As) is a metalloid found throughout the environment. Although As can be released from natural phenomena, anthropogenic activities account for most As contamination worldwide. The toxicity of As depends on the form (inorganic or organic) and species (AsIII or AsV), among others. Plants have the ability to absorb and bioreduce As, cleaning the soil and reducing the toxicity of As to some extent. The aim of the present research was to study the effects of cysteine, the chelating agents cyclohexylenedinitrotetraacetic acid and nitrilotriacetic acid, and the phytohormone kinetin on the As concentration and speciation in mesquite (Prosopis sp.). The results give an insight about how a desert plant absorbs, bioreduces, distributes and stores this toxic metalloid. Abstract. The aim of the present research was to study the effects of cysteine (Cys), cyclohexylenedinitrotetraacetic acid (CDTA), nitrilotriacetic acid (NTA), and kinetin (KN) on the arsenic (As) concentration and speciation in mesquite (Prosopis sp.) grown in soil containing 30 ppm (parts per million) of AsIII or 50 ppm of AsV. Inductively coupled plasma–optical emission spectroscopy (ICP-OES) determinations revealed that, compared with As alone, roots of plants treated with 2.5 mM CDTA or 0.5 mM of Cys + 100 µM KN increased total As concentration from AsIII by ~20 and 36% and from AsV by 100 and 65%, respectively. Liquid chromatography–inductively coupled plasma–mass spectrometry (LC-ICP-MS) studies revealed that in roots, AsIII remained without change, whereas both AsIII and AsV were found in plants grown with AsV. X-ray absorption spectroscopy (XAS) studies revealed that As within plants was mainly coordinated to three sulfur atoms, with interatomic distances of 2.26 Å. Results suggests that Cys + KN increased the mesquite tolerance to AsV, because plants grown in AsV had roots of similar size to plants grown without As.