Eu‐doped MgAl2O4 has been used to evaluate the kinetics of equilibrium grain boundary transformations, otherwise known as complexion transitions, by monitoring abnormal grain growth induced the nucleation of highly mobile complexions. The assumption in prior works was that abnormal grain growth can be charted using time‐temperature‐transformation (TTT) diagrams to reflect the complexion transition nucleation and growth kinetics. A model depending on doping concentration, grain size, and abnormal area fraction has also been recently developed to estimate excess grain boundary coverage, and thereby predict complexion types depending on microstructural descriptors. While useful, the grain boundary excess model has not been validated using atomic‐resolution characterization. This work aimed to directly validate the grain boundary excess model and complexion TTT diagrams by applying aberration‐corrected electron microscopy to characterize grain boundary structures and compositions (i.e., complexions) in Eu‐doped MgAl2O4. Eu doping concentrations of 100 ppm and 500 ppm were produced and bulk samples were annealed at 1600°C for different times. Forty‐five distinct grain boundaries were characterized. Overall, at least five grain boundary complexion types were identified and the grain boundary excess model was validated. Interpretability of the grain boundary excess model and correlations between grain boundary structures and compositions are discussed.