Diodes of configuration HxWO3‖p‐ orn‐Si and HxWO3‖n‐CdS have been fabricated, where the effective work function of the HxWO3can be varied reversibly by electrochemical hydrogen insertion/extraction. The diodes have a continuously variable barrier height, as indicated by their saturation photovoltage. These systems serve as prototypes for a class of chemically sensitive electronic devices.