In this work manganese oxide films were obtained by electrodeposition and their electrochemical and mass transfer processes in aqueous zinc-ion battery electrolyte were studied by cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM). Cyclic voltammograms and corresponding mass variation curves of manganese oxide during charge-discharge processes were examined simultaneously on Au-coated quartz crystal electrodes. The investigations were conducted in aqueous electrolytes of different composition (2 M ZnSO ₄ and 2 M ZnSO ₄ + 0.1 M MnSO ₄). Monitoring of electrode mass variation during potential cycling provides direct evidence that redox processes in MnO ₂ electrodes co-occur with intercalation of protons and zinc ions. Combined CV and EQCM studies reveal that electrodeposited films of MnO ₂ are unstable in 2 M ZnSO ₄ electrolyte. The repeated potential cycling in Zn-containing electrolytes leads to rapid deterioration of electrode capacity in the few initial cycles due to the Zn ²⁺ insertion into subsurface structures of MnO ₂ and blocking of electroactivity of MnO ₂ film on Au substrate. On the other hand, reversible processes of intercalation of protons and zinc ions occur in 2 M ZnSO ₄ + 0.1 M MnSO ₄ electrolyte. Two main steps of mass increase during the discharging process, taking place at 1.4 V (vs. Zn/Zn ²⁺) and in the potential range (1.3–1.0) V were demonstrated by EQCM. The first step of mass increase is mainly related to the intercalation of H ⁺ (as H ₃O ⁺), whereas the second step of mass increase is mainly associated with formation of surface compounds like zinc sulfate hydroxide salts.