The densification and grain growth of the solid state ionic conductor material Ce0.9Gd0.1O1.95−δ (i.e. GDC10, gadolinium-doped ceria, with Gd 10 mol.%) are analysed for nanometric and fine powders of various particle sizes, both in air and in a 9 vol.% H2–N2 mixture. Due to a dominant solute drag effect in aliovalent highly doped ceria, the starting morphology of the powders controls the diffusion mechanisms of the material in air. Conversely, highly enhanced densification and grain growth are achieved by firing the materials at reduced temperatures (800 < T < 1200 °C) in low oxygen activity atmospheres (pO2 < 10−12 atm). Solute drag is not the rate-limiting step in highly defective GDC and the densification mechanisms are nearly independent of the starting powder properties. Fast diffusion is activated under low oxygen activity with high grain boundary mobility (e.g. Mgb ∼ 10−10 m3 N−1 s−1 at 1100 °C). The change of the dominant sintering mechanisms under low oxygen activity is attributed to the formation of a large concentration of oxygen vacancies (View the MathML source), electronic defects (View the MathML source, i.e. Ce3+) and reduced Gd/Ce cation mismatch. High densification and electric conductivity are achieved in Ce0.9Gd0.1O1.95−δ at low temperatures (∼1000 °C) and low oxygen activity, preserving the mechanical integrity of the material.