In this work, the stability of NASICON in contact with water was studied by impedance spectroscopy, structural and microstructural analyses, using both dense and powder samples. Impedance spectroscopy data showed a significant decrease in ionic conductivity of dense pellets when exposed to water for more than 300 h. While both grain and grain boundary resistivities increased, the major contribution and changes in the overall impedance spectra were due to changes in the grain boundary are. Based on complementary microstructural analysis, small grains of a second phase were found covering large grains of NASICON. The composition of these small grains is believed to be similar to that of hydronium NASICON, identified by XRD after the reaction of NASICON powder with water.