Chemical behaviours of arsenate and arsenite in original (undisturbed) soil solutionand their phytotoxicity to rice (Oryza sativa L. ) were investigated in a pot experiment. Meanwhile, a laboratory study on equilibrium adsorption of As was conducted to explain the resultsfrom the pot experiment. It was found that critical Eh, i. e., Eh0, of redox in As (Ⅱ) As (Ⅴ) system was between 100 mV and 200 mV in the paddy soil studied. Concentration oftotal as and the added As species, As (Ⅲ) or As (Ⅴ), in the soil s...