The oxygen nonstoichiometry (delta) of Ce0.8Pr0.2O2-delta has been measured as a function of P-O2 at temperatures between 600 and 900 degrees C by coulometric titration and thermogravimetry. An ideal solution defect model, a regular solution model, and a defect association model, taking into account the association of reduced dopant species and oxygen vacancies, were unable to reproduce the experimental results. However, excellent agreement with the experimentally determined oxygen nonstoichiometry could be achieved when using either a nonideal solution model with an excess enthalpic term linear in delta (Delta H-Pr(exc)=a(H)delta) and a completely random distribution of defects (referred to as "delta-linear"), or a "generalized delta-linear" solution model, where the excess Gibbs energy change in the reduction reaction of the dopant linearly varies with delta (Delta G(Pr)(exc)=a(G)delta). A comparison of the partial molar enthalpy and entropy of oxidation, estimated from the defect models with those determined directly from the oxygen nonstoichiometry, suggests that the delta-linear solution model is the most appropriate in accounting for the observed nonideal reduction behavior of Pr.