Oxygen nonstoichiometry of Ce0.8Gd0.2O1.9-chi, was gravimetrically measured at temperatures from 700 to 1000 degrees C and oxygen partial pressures between 10(-24) and 1 atm. Based on the experimental data, the relationship between log chi and log po(2), (where chi is the oxygen deficiency and po(2) the oxygen partial pressure) is discussed and partial molar enthalpy, Delta H-o, and entropy, Delta (S) over bar o, of oxygen are calculated. In the region of chi less than or equal to 0.04, chi is proportional to the -1/4 power of po(2) and Delta H-o, and Delta (S) over bar(o), behave as in an ideal solution, showing that oxygen vacancies are doubly charged and defects in Ce(0.8)Gd(0.2)O(1.9-chi)are randomly distributed. Thus, the mass action law is satisfied in the area chi less than or equal to 0.04.