Long-term stability of a dc voltage standard based on a capacitive microelectromechanical system (MEMS) is investigated. Square-wave voltage actuation was used to cancel the effect of slow charging appearing in the dielectric layers and capacitor plate surfaces of the MEMS capacitor. The key properties of a MEMS capacitor and the operational principle and the measurement results of the apparatus that implements the voltage standard are explained. Negligible voltage drift within the measurement scatter of ± 2 parts per million during a 22-day measurement period was demonstrated.