Electrodeposition is an important approach that can produce functional compound materials by assembling multiple species at the electrode surface. However, a fundamental understanding of the electrodeposition mechanism has been limited by its complexity and is often gained only through ex situ studies of deposited materials. Here we report on the application of scanning electrochemical microscopy (SECM) to enable the in situ, real-time, and quantitative study of electrodeposition and electrodissolution. Specifically, we electrodeposit magnetite (Fe3O4) from an alkaline solution of Fe(III)-triethanolamine as a robust route that can prepare this magnetic and electrocatalytic compound on various conductive substrates. The powerful combination of SECM with cyclic voltammetry (CV) at a gold substrate reveals that the electrodeposition of magnetite requires the preceding adsorption of Fe(II)-triethanolamine on the substrate surface and, subsequently, is mediated through the highly complicated ECadsCmag mechanism, where both chemical steps occur at the substrate surface rather than in the homogeneous solution. SECM-based CV is obtained under high mass-transport conditions and analyzed by the finite element method to kinetically resolve all steps of the ECadsCmag mechanism and quantitatively determine relevant reaction parameters. By contrast, the adsorbed Fe(II) intermediate is unresolvable from co-deposited magnetite in situ by other electrochemical techniques and is undetectable ex situ because of the facile air oxidation of the Fe(II) intermediate. Significantly, SECM-based CV will be useful for the in situ characterization of various electrodeposited compounds to complement their ex situ characterization.