Coldfusion of deuterium by electrolysis of heavy water onto a palladium (or titanium) cathode has been reported. Contrary to the assumption of Fleischmann and Pons that electrochemically compressed D+ exists inside the palladium cathode, the observations of Jones et al. can be partially explained by the simultaneous presence of deuteride D− and the highly mobile positive deuterium ion D+. The opposite charges reduce the intranuclear distance and enhance the tunneling fusion rate. Furthermore alloying of lithium with palladium can stabilize a negatively charged deuteride ion due to the salinelike character of lithium deuteride. The enormous pressure (or fugacity), achieved by the applied electrochemical potential (1030 atm), is a virtual pressure that would have existed in equilibrium with palladium deuteride (PdDx). It is speculated that nuclear fusion occurs at the surface, and the PdDx serves as a reservoir for the supply of deuteride ions.