Using scanning transmission electron microscopy we image similar to 4 nm platinum nanoparticles deposited on an insulating membrane, where the membrane is one of two electron-transparent windows separating an aqueous environment from the microscope's high vacuum. Upon receiving a relatively moderate dose of similar to 10(4) e/nm(2), initially immobile nanoparticles begin to move along trajectories that are directed radially outward from the center of the field of view. With larger dose rates the particle motion becomes increasingly dramatic. These observations demonstrate that, even under mild imaging conditions, the in situ electron microscopy of aqueous environments can produce electrophoretic charging effects that dominate the dynamics of nanoparticles under observation.