Geothermal arsenic is an important source of arsenic in the environment, and methylthiolated arsenates may constitute a non- negligible fraction of arsenic species in geothermal waters. Compared to non- thiolated and/or non- methylated arsenic species, methylthiolated arsenic species are generally characterized by higher toxicity and stronger mobility. However, their occurrence in natural water environments, including diverse geothermal settings, remains relatively understudied. Two theoretical formation pathways exist for methylthiolated arsenic species. The first, involving methylation of arsenite followed by thioltion, has been well acknowledged. In contrast, the second pathway, where arsenite undergoes thioltion prior to methylation, lacks validation primarily due to the absence of an effective quantitative analytical method for thioarsenites. Nevertheless, the detection of methylthiolated arsenates in neutral to alkaline hot springs, which are fed by rapidly replenished high- temperature shallow reservoir fluids, indicates that the second pathway may be more plausible in these cases. Once formed in a hot spring area, methylthiolated arsenates tend to show a lower adsorption affinity to hot spring sediments that are capable of strongly adsorbing other arsenic species. Consequently, they can undergo long- distance transport via rivers traversing the area, participating in a larger- scale arsenic geochemical cycle. Further research into the transport and fate of methylthiolated arsenic species in geothermal areas, as well as their environmental effects at regional and global scales, is crucial and represents a key research direction in arsenic environmental geochemistry.