Kinetic and thermodynamic studies between the ordered ι ( n = 7 in Pr n O 2 n−2 ) and the disordered α (PrO 2− x ) phases have been carried out as a function of oxygen pressure at 655, 675, 695, and 715°C using a sample of small single crystals. The existence of a reproducible hysteresis loop, which depends on the temperature and pressure, having an inflection point around a composition PrO 1.75 is shown. The inflection point is interpreted as a phase of n = 8 in Pr n O 2 n−2 (η) coherently intergrown with n = 7, 9, and 10, giving an average composition of PrO 1.75. In order to interpret the kinetic data, various theoretical models were examined, for example, oxygen diffusion, a phase-boundary reaction control, or nucleation and growth. None of these models, however, is capable of correlating the experimental data. It was found that a plot of the reaction rate versus the ambient oxygen pressure extrapolates linearly to a finite pressure at zero rate, as was previously observed in the oxidation reaction between the ι and ζ phases of the same system. The model developed for the oxidation reaction between the ι and ζ phases has been modified by taking into account the intermediate phases around PrO 1.75, through which the reaction passes. The modified rate law has two reaction constants: the rate constant from the reactant transforming to the intermediate phase ( k 1) and for the transformation from the intermediate to the product phase ( k 2). The fit to the experimental curve is satisfactory for both the oxidation and reduction reaction. From the temperature dependence of the observed rate constants, the activation energy for the oxidation and reduction was determined to be 75.0 and 60.9 kcal/mole, respectively.