The ionic strength in supporting electrolyte solution had a significant influence on the electrochemical and electrocatalytic behaviors of myoglobin (Mb) in {HA/Mb}(n) films, which were assembled layer-by-layer on pyrolytic graphite (PG) electrodes with oppositely charged hyaluronic acid (HA) and Mb. The results of cyclic voltammetry (CV), quartz crystal microbalance (QCM), scanning electron microscopy (SEM), rotating disk voltammetry (RDV), and electrochemical impedance spectroscopy (EIS) showed that after incubation with testing solution at high concentration of salt (C-KCl), the {HA/Mb}(n) films swelled and the film permeability was enhanced, suggesting that the external salt ions and accompanied water molecules in the exposure solution are incorporated into the films. Systematic investigation of the type and size effect of counterions in supporting electrolyte solution on the electrochemical responses for the {HA/Mb}(n) films and the positive shift of the formal potential (E degrees') with C-KCl suggest that it is cationic rather than anionic counterions that control the electrode process of {HA/Mb}(n) films at PG electrodes with electron hopping mechanism. The salt-induced swelling of {HA/Mb}(n) films facilitated the transportation of counterions, and then accelerated the electron transfer of Mb in the films with the underlying electrodes, making the film electrodes show better CV responses. The comparative study showed that only Mb layer-by-layer films assembled with "soft" and flexible polyions could demonstrate the salt-induced effect and that the {HA/Mb}(n) films showed better swelling capability than {PSS/Mb}(n) films (PSS = poly(styrenesulfonate)) due to the unique character of HA.