Using a movable top electrode, for the first time, top vs. bottom charging of the dielectric in metal/insulator/metal capacitors is delineated. For the Al/SiO₂/Cr structure used in RF MEMS capacitive switches, charge injection from Al into the top of SiO₂ was found to have a higher threshold voltage, faster charging time, and slower discharging time than charge injection from Cr into the bottom of SiO₂. The higher threshold is attributed to non-ideal contact geometry and chemistry. The faster charging time is attributed to the exponential voltage dependence. The slower discharging time is attributed to diffusion across SiO₂. Since top charging is more critical to switch performance and reliability than bottom charging, understanding the trade off of top vs. bottom charging can help minimize their undesirable effects.