Potentiostatic polarization and rotating disk electrode techniques were used to obtain the rate constant for the dissolution of electrochemically-formed (at 1 V) WO3 on tungsten (W) in acidic solutions. The corresponding rate constant for the chemical dissolution of WO3(s) powder was found by measuring the dissolved tungsten concentration as a function of time and pH. The chemical dissolution experiments supported the view that the rate-determining step in the anodic reaction of W in acidic solution is the chemical dissolution of WO3(s) formed on the metal surface. Zeta potential measurements gave the isoelectric point (iep) of the WO3(s) powder as pH 1.5, a value that was somewhat smaller than the point of zero charge (pzc) of WO3(s) formed on W metal (pH 2.5). This difference was attributed to the highly hydrated nature of the oxide film formed on W metal in aqueous systems.