Sorption mechanisms for the binding of Arsenic (As) to thiols (-SH) in natural organic matter (NOM) plays an important role in controlling inorganic As mobility and bioavailability in anaerobic sulfidic environments. The determination of reversibility of Arsenite (As(III)) sorption onto the thiol-functionalized ion exchange resin Ambersep GT74 has not been studied, despite constant interest in the dynamics of inorganic As species in wetland and paddy field environments due to concerns over As uptake into rice. This contribution investigates reversible sorption of As(III) onto Ambersep GT74 as a model for thiols in NOM to determine the sorption isotherm parameters for As(III) binding to Ambersep GT74, assess the effects of ionic strength and pH on As(III) sorption, and evaluate whether reversibility or irreversibility can best characterize As(III) sorption. Through a combination of potentiometric titration analyses, adsorption and desorption isotherm experiments, kinetic modeling, and ionic strength experiments, we determined the proton dissociation constant (pKa) and thiol binding site density of Ambersep GT74, identified the best fit isotherm and kinetic models for As(III) sorption onto Ambersep GT74, and assessed the reversibility of sorption reactions. The thiol binding site density was found to be 1.79 mmol/g ambersep and pKa of thiol sites was determined to be 9.6. Langmuir isotherm model fits indicated that affinity constants of Ambersep GT74 for As(III) were similar at pH 5.4, 6.5, and 8. Furthermore, the adsorption kinetics were well described by the Elovich equation. Lastly, desorption reactions following adsorption confirmed that irreversible As(III) sorption was occurring, thereby suggesting a low amount of As(III) being bioavailable for uptake and less As(III) mobility.