The activity coefficients of NaClO4 in water were experimentally determined at (288.15, 298.15, and 308.15) K from electromotive force (emf) measurements of the following electrochemical cell without transference containing two ion-selective electrodes (ISEs): Na-ISE vertical bar NaClO4 (m)vertical bar ClO4-ISE. The molality m varied between about (0.02 and 12) mol.kg(-1). The determination of E-0*, which is necessary to calculate the activity coefficients, was carried out following a method similar to that of Hitchcock, using the Pitzer, extended Debye-Huckel, and Scatchard equations to represent the dependency of log gamma on molality. The electrode system employed shows good Nernstian behavior. The activity and osmotic coefficients obtained in this study at 298.15 K were contrasted to those from the literature and show a good agreement.