D 2O electrolysis from 0.1 M LiOD solutions was carried out; in a cell both with and without diaphragm, under conditions similar to those used by Fleischmann et al. (J. Electroanal. Chem., 261 (1989) 301; errata, 263 (1989) 187). The following quantities were followed electrolysis parameters: energy balance of the electrolysis in the form of resulting excess heat, individual gas evolution rates, cathodic formation of PdD n , and also, in the case of the divided cell, the gas purity. Within the experimental accuracy of 0.5%, no excess heat was determined during the electrolysis in either type of electrolytic cell. In this respect, D 2O electrolysis with a Pd cathode behaves just like normal H 2O electrolysis. In the diaphragm cell, in the stationary state of electrolysis, the current yields of the two gas products were equal to 1.0. In the initial stage of electrolysis, cathodically discharged deuterium was adsorbed in the palladium cathode under PdD n , formation, until a stationary composition of PdD n , had been achieved. In the cell without diaphragm, during the initial non-stationary course of electrolysis, part of the primary discharged deuterium was absorbed in the palladium cathode, another part recombined with anodically formed oxygen back to D 2O(liq). The fraction of the D 2 + O 2; mixture undergoing the recombination reaction diminished markedly with increasing deuterium content in the PdD n , cathode. In the stationary state of electrolysis, when no further deuterium absorption takes place (i.e. for n ~ 0.7 to 0.8), the stationary extent of the recombination dropped to nearly zero. After 270 h of electrolysis, a platinum deposit was determined on the surface of the Pd cathode, apparently originating from the partially corroded Pt anode. This Pt deposit catalyses the recombination reaction, even on the surface of practically inhibited PdD n .