It is known that Fe (III) and As (III), through in situ oxidation, can precipitate to form a crystalline ferric arsenate under specific conditions of temperature and pH. A thermodynamic model is built to investigate this process. The development and the use of chemical speciation model have lead to simulate the distribution and the concentration of chemical species of the ferric-arsenate (III)-sulfate system at 293.15-553.15 K temperatures range. The modeling predictions are compared to experimental data measured by the authors. It was shown that a reasonable prediction could be made if different modeling approaches are used. The paper compares different approaches in thermodynamic modeling and discusses also the validity of different extrapolation models used at high temperatures. (C) 2003 Published by Elsevier Science Ltd.