Highly sensitive laser interferometer was built to measure electromechanical coupling in Gd-doped ceria Ce0.9Gd0.1O2-x thin films in the frequency range up to 20 kHz. Spurious resonances due to substrate bending were avoided by the special mounting of the film in the center of substrate. Compact design allowed to reach high vertical resolution of about 0.2 pm. Electrostriction coefficient measured in 1 µm thick Ce0.9Gd0.1O2-x film was 4.3×10-21 m2/V2 and slightly decreased with frequency till the extensional resonance of the substrate at about 20 kHz occurred. As expected, the displacement varied as a square of applied voltage without any sign of saturation.