AbstractFunctional properties of mixed ionic electronic conductors (MIECs) can be radically modified by (de)insertion of mobile charged defects. A complete control of this dynamic behavior has multiple applications in a myriad of fields including advanced computing, data processing, sensing or energy conversion. However, the effect of different MIEC's state‐of‐charge is not fully understood yet and there is a lack of strategies for fully controlling the defect content in a material. In this work we present a model‐less technique to characterize ionic defect concentration and ionic insertion kinetics in MIEC materials: Iono‐Optic Impedance Spectroscopy (I‐OIS). The proof of concept and advantages of I‐OIS are demonstrated by studying the oxygen (de)insertion in thin films of hole‐doped perovskite oxides. Ion migration into/out of the studied materials is achieved by the application of an electrochemical potential, achieving stable and reversible modification of its optical properties. By tracking the dynamic variation of optical properties depending on the gating conditions, I‐OIS enables to extract electrochemical parameters involved in the electrochromic process. The results demonstrate the capability of the technique to effectively characterize the kinetics of single‐ and even multi‐layer systems. The technique can be employed for studying underlying mechanisms of the response characteristics of MIEC‐based devices.