ThAs2 was reported to crystallize in the tetragonal Cu2Sb or C38 type of structure1. In an attempt to synthesize this phase by reacting arsenic with thorium powder at temperatures between 600° and 900° C, a new modification was obtained. This low-temperature form α-ThAs2 is orthorhombic and of the C23 type. Its lattice constants are: The same structure type is also found in ZrAs2 (ref. 2), HfAs2 (ref. 3), TiP2, ZrP2 and HfP2 (ref. 4). The transition to the tetragonal high-temperature modification β-ThAs2 appears to occur at approximately 1,100° C. A sample held at 1,150° C for one day was in parts converted into the C38 modification.