Arsenic is known to be one of the most toxic inorganic elements, causing worldwide environmental contamination. However, many fundamental properties related to aqueous arsenic species are not well known which will inhibit our ability to understand the geochemical behavior of arsenic (e.g. speciation, transport, and solubility). Here, the electrical conductivity of Na₂HAsO₄ solutions has been measured over the concentration range of 0.001–1 mol kg⁻¹ and the temperature range of 5–90°C. Ionic strength and temperature-dependent equations were derived for the molal conductivity of HAsO₄²⁻and H₂AsO₄⁻ aqueous ions. Combined with speciation calculations and the approach used by McCleskey et al. (2012b), these equations can be used to calculate the electrical conductivities of arsenic-rich waters having a large range of effective ionic strengths (0.001–3 mol kg⁻¹) and temperatures (5–90°C). Individual ion activity coefficients for HAsO₄²⁻ and H₂AsO₄⁻ in the form of the Hückel equation were also derived using the mean salt method and the mean activity coefficients of K₂HAsO₄ (0.001–1 mol kg⁻¹) and KH₂AsO₄ (0.001–1.3 mol kg⁻¹). A check on these activity coefficients was made by calculating mean activity coefficients for Na₂HAsO₄ and NaH₂AsO₄ solutions and comparing them to measured values. At the same time Na-arsenate complexes were evaluated_. The NaH₂AsO₄⁰ ion pair is negligible in NaH₂AsO₄ solutions up to 1.3 mol kg⁻¹. The NaHAsO₄⁻ ion pair is important in NaHAsO₄ solutions >0.1 mol kg⁻¹ and the formation constant of 10^0.69 was confirmed. The enthalpy, entropy, free energy and heat capacity for the second and third arsenic acid dissociation reactions were calculated from pH measurements. These properties have been incorporated into a widely used geochemical calculation code WATEQ4F and applied to natural arsenic waters. For arsenic spiked water samples from Yellowstone National Park, the mean difference between the calculated and measured conductivities have been improved from −18% to −1.0% with a standard deviation of 2.4% and the mean charge balances have been improved from 28% to 0.6% with a standard deviation of 1.5%.