An electrochemical process for producing crystalline nanoparticles of a mixed metal oxide, mixed metal hydroxide or mixed metal oxide hydroxide reaction product containing at least a first and a second metal ion which are different from each other, the method comprising the steps of
(1) supplying to a cathode compartment of an electrochemical cell equipped with a gas diffusion cathode, a liquid feed electrolyte solution containing a first precursor salt of the first metal ion in a first concentration and a second precursor salt of the second metal ion in a second concentration to obtain a reaction mixture;
(2) supplying an O 2 containing oxidant gas to the cathode,
(3) subjecting the cathode to an electrochemical potential which is below the thermodynamic limit of O 2 reduction at the pH of the reaction mixture,
(4) applying a potential to the electrode to cause reduction of the O 2 contained in the oxidant gas to one or more of the corresponding peroxide, OH - , ionic and/or radical reactive O containing species,
(5) wherein a ratio R Q of charge Q applied to the cathode relative to the sum of the individual concentrations of the at least one first and second metal ion ([M 1 ] + [M2]) ranges between 100 and 1500 C/mmol, wherein [M 1 ] is the concentration of the first metal ion in the reaction mixture and [M2] is the concentration of the second metal ion in the reaction mixture, and isolating nanoparticles of the reaction product.