The Young’s modulus (E), hardness (H) and fracture toughness (KIC) of various compositions of gadolinia doped-ceria (GDC, GdxCe1−xO2−x/2, 0.1 ⩽ x ⩽ 0.2) and yttria-stabilized zirconia (YSZ, Y0.08Zr0.92O1.96) electrolytes were investigated by nanoindentation. All samples were produced by the sol–gel method, formed by uniaxial pressure and sintered at 1400 °C. In order to determine the mechanical properties, a Berkovich diamond tip was employed at applied loads of 5, 10, 30, 100 and 500 mN. The results were interpreted by the Oliver–Pharr method and values of KIC were determined using the method of Palmqvist cracks. The residual imprints were observed by field emission scanning electron microscopy. The results obtained showed that the H, E and KIC of GDC decreased with increasing gadolinia concentration, due to the oxygen vacancies generated by the dopant addition. As a result, the mechanical properties of GDC were significantly lower than those of YSZ electrolyte.