There is a need to identify alternative compounds for arsenic remediation that exhibit higher phase stability than those generated using conventional methods. The first step in assessing the possibilities is through chemical-thermodynamic simulations. It is known that some rare earth arsenates are highly stable, however, rare earth element (REE) arsenate thermodynamic data are limited. Hence, an overview of lanthanum (La), neodymium (Nd), and cerium (Ce) chemical stability was performed under different systems scenarios (i.e., aqueous only, arsenate, sulfate, or chloride). It is found that the REE arsenate is stable over a broad pH range, and incongruent dissolution or phase transformation (e.g., hydroxide) is expected at highly alkaline conditions. Chloride did not influence the solubility of the arsenates, whereas sulfate increased solubility in the region where LaSO4+ and NdSO4+ species are predominant (pH < 10). Metastability simulations were not considered, and the phases were compared regarding their equilibrium thermodynamic stability alone. Iron (Fe) is commonly used as a source for arsenic removal and has been widely studied. It was used here as a baseline for comparison with La and Nd arsenate stability. An experimental investigation into REE arsenate stability is currently underway, and a more detailed discussion and evaluation will be published in the near future.