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|NaClO4 (m)|ClO4-ISE. The molality m varied between about (0.02 and 12) mol·kg-1. The determination of E0, which is necessary to calculate the activity coefficients, was carried out following a method similar to that of Hitchcock, using the Pitzer, extended Debye-Hückel, and Scatchard equations to represent the dependency of log γ 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.