The speciation of arsenic (III) and (V) in aqueous acidic and basic solutions at ambient conditions, and of arsenic (V) in acidic solutions at 270 bars from 30 to 300 °C was investigated using in situ X-Ray Absorption Spectroscopy (XAS). Both Extended X-Ray Absorption Fine Structure (EXAFS) analysis and X-Ray Absorption Near Edge Structure (XANES) ab initio calculations were used to determine the structure of dominant arsenic species in solution. At ambient conditions, the deprotonation of AsIII(OH)3 and AsVO(OH)3 species with increasing pH is demonstrated by analysis of their XANES signals, in agreement with available arsenious and arsenic acid dissociation constants. In basic conditions, the dominant species is a deprotonated form of the As(OH)3 acid, with a very similar symmetry. The speciation of AsV in acidic hydrothermal fluids is dominated by the AsVO(OH)3 complex over a wide temperature and pressure range, to at least 300 °C and 300 bars. The first-shell As structure in the AsVO(OH)3 species and its deprotonated forms is found to be a distorted tetrahedron, similar to the base AsO4 unit of Na2HAsO4·7H2O(s). This work demonstrates new capabilities of XAS coupled with molecular modelling to decipher deprotonation reactions that affect mineral solubilities and chemical elements mobility in aqueous crustal fluids.