Potassium polyferrites K2 $${\text{Fe}}_{{1 - q}}^{{{\text{II}}}}$$ Mq $${\text{Fe}}_{{10}}^{{{\text{III}}}}$$ O17 (where M is Mg and Zn) and K2–2qFeIIMq $${\text{Fe}}_{{10}}^{{{\text{III}}}}$$ O17 (where M is Ca and Sr; q = 0–0.4) are synthesized to determine the effect of the size of a doubly charged dopant ion on their catalytic properties. The mechanism of the effect of doubly charged cation additives on the activity, selectivity, and corrosion resistance of potassium β'' polyferrite is identified. It is shown for the first time that doubly charged cations are distributed differently in the structure of β'' polyferrite depending on their size: either they are in a spinel-like block and then replace Fe2+ ions, or they replace potassium in interblock space. The Mg2+ and Zn2+ ion additives sharply reduce the catalytic activity and selectivity of β'' polyferrite. Such additives are undesirable. Small amounts of Ca2+ and Sr2+ cations greatly improve the corrosion resistance of the catalysts due to a drop in the mobility of potassium ions within a cation-conducting layer.