The transformation of iron sulfide to pyrite is an important process in anoxic environments that strongly influences the mobility of metal(loid)s such as As. It is, therefore, important to understand the fate of As in this process. In this study, batch experiments were conducted at 150 degrees C and 80 degrees C and pH 7.5 and 6.0 to examine the transfer of As in the transformation of amorphous FeS to pyrite. The solid phase was characterized using XRD, SEM-EDS, and XPS. The transformation of FeS to pyrite can be promoted in the presence of As(III). During the transformation of FeS loaded with As(III), dissolution of FeS occurs prior to pyrite formation. As(III) is first released in the stage of FeS dissolution, and then is incorporated into the newly formed pyrite. Both high temperature and low pH can promote As uptake by pyrite. XPS analysis indicated sorption as a pathway of As incorporation into pyrite. The contribution from the pathway of lattice substitution is proposed to become significant at high temperature. The results of this study show that As lost or gain in solid phases can occur depending on the As uptake ability by pyrite under different conditions and the initial As loading.