The electrochemically-controlled uptake and release of Cs+ by nickel hexacyanoferrate thin-film electrodes that were prepared with different counter cations (Li+, Na+, K+, Rb+, or Cs+) were examined using a thin-layer flow cell coupled on-line with electrospray-mass spectrometry (ES-MS). A potential step technique was used to control the uptake and release of the cations by the films. Performance of the different films in terms of Cs+ capacity, Cs+ uptake selectivity versus the other alkali metal ions, film stability, and the effect of a saline matrix on Cs+ uptake were examined. These results show that the electrochemistry-mass spectrometry (EC-MS) combination provides rapid, Cs+ specific information concerning the films that is similar to or complementary to that information which might be obtained using conventional electrochemical approaches. Information that cannot be obtained electrochemically (e.g. isotopic distributions of metal ions accumulated and released) might also be gathered. Any number of electrode reactions involving ion uptake and release might be studied using this approach.