Grain boundaries are known to block ionic conduction across grain boundaries in oxide ion conductors due to adjacent space charge layers. Since the positively charged grain boundary core is intensified with a high local concentration of defects such as oxygen vacancies, uniform distribution of a dopant may mitigate the formation of space charge layers and enhance the ionic conductivity. To investigate the dopant segregation effect on the space charge layer and ionic conductivity, we provided thermal energy to nanocrystalline gadolinia-doped ceria (GDC) thin film by post-annealing at different temperatures of 700 degrees C, 900 degrees C, and 1100 degrees C. STEM-EELS analysis demonstrates strong dopant segregation and a higher Ce3+ content near the grain boundary than in the bulk after post-annealing. The concurrent segregation of dopants and Ce3+ ions implies that once thermal treatment is applied to nanocrystalline GDC thin films, complete space charge layers are formed while the non-thermally treated GDC film with a uniform distribution of dopants has less of a space charge effect and exhibits superior ionic conductivity. (C) 2016 The Electrochemical Society. All rights reserved.