We report the first dynamic in situ electrochemical neutron reflectivity (NR) measurements on electroactive films. By using a boxcar integration strategy within a cyclic voltammetric experiment, it is possible to acquire neutron reflectivity data associated with narrow, defined windows of potential. Accumulation of data from repetitive cycles allows one to build up potential- (time-) resolved profiles. The effective time resolution is now on the order of seconds, as compared to ca. 1 h using conventional methodology, making in situ NR a practical technique for dynamic electrochemical studies. Illustrative data for polyvinylferrocene films reveal hysteresis in (de)swelling, incomplete desolvation upon reduction, and transient salt retention, all of which respond to time-scale variations.