Adsorption isotherms are reported for pure water vapor on zeolite 13X (also called zeolite NaX) pellets. Data were obtained using a gravimetric method over a temperature range of 25 to 100 degrees C and a pressure range of 0.006 to 25 kPa. These pure-component equilibria are fit with the Sips, Toth, and multisite Langmuir models, all modified with the Aranovich-Donohue (A-D) model. The A-D Sips isotherm is recommended for modeling water adsorption on zeolite 13X because it provides the best agreement with the measured isotherm data.