Electrical conductivity relaxation (ECR) and oxygen permeation measurements were conducted, at 750 °C, to assess the long-term oxygen transport characteristics of the mixed ionic-electronic conducting Pr2NiO4+δ with a K2NiF4 structure. The results show that the apparent values for the oxygen diffusion and surface exchange coefficients extracted from the data and the associated oxygen flux increase over 120 h by 1-2 orders of magnitude. The results of post-mortem X-ray diffraction analysis of the samples show partial to virtually complete decomposition of Pr2NiO4+δ under the conditions of the experiments to Pr4Ni3O10+δ, PrNiO3-δ, Pr6O11, and traces of NiO. Pulse 18O-16O isotopic exchange (PIE) measurements confirmed fast surface exchange kinetics of the higher-order Ruddlesden-Popper phase Pr4Ni3O10+δ and Pr6O11 formed upon decomposition. Additional factors related to the microstructure, however, need to be considered to explain the observations.