Previous solubility studies of absorbed gaseous iodine by Ag7PO4I4 (API) in a wet atmosphere showed that API absorbs a great quantity of iodine (up to 1.8 mol of iodine per mol of API). The absorption rate law depends on the square root of iodine fugacity plus a constant term that represents a threshold fugacity (f(I2)(o), = 0.4 +/- 0.5 Pa) for the iodine absorption process. In this work, thermodynamic measurements carried out on the system {API + I-2} by using solid-state galvanic cells, gives a value (p(I2) = (1.2 +/- 0.7) x 10(-2) Pa at 298 K) consistent with the one previously obtained with kinetic measurements. The API compound is stable for activity values lower than the mentioned fugacity. (c) 2005 Elsevier B.V. All rights reserved.