This is the first of a two-part paper that is concerned with measuring and interpreting equilibrium and transient conductivity for 10 at.% gadolinium-doped ceria (Ce0.9Gd0.1O1.95-δ, GDC10). AC impedance spectroscopy is used to measure conductivity of slender extruded GDC10 rods at temperatures between 700 and 900 °C. From the obtained Brouwer diagrams, the activation energies of the oxide ions and cerium small polarons are estimated to be 0.77 and 2.45 eV respectively. Conductivity relaxation measurements from strongly reducing conditions (4% H2, 3% H2O, 93% Ar) to strongly oxidizing conditions (97% O2, 3% H2O) and vice versa reveal great asymmetries in relaxation times. Virtually instantaneous relaxations are observed in the first case, while the oxidizing-to-reducing relaxations take as long as 0.5 h. This paper reports conductivity relaxations with over 20 orders of magnitude in the gas-phase oxygen partial pressure. Models and experiments are used to show that the observed asymmetries cannot be explained by electrode defects or two-dimensional end effects. The companion paper develops a Nernst–Planck–Poisson model to assist the quantitative interpretation of the measurements reported in the present paper.