The mode of deuterium uptake during Pd-D co-deposition has been explored using galvanostatic perturbation techniques. The resultant potential relaxation curves exhibit four distinct potential-time intervals where the relaxation process is controlled by the interaction between the transport of deuterium from the lattice to the surface to form adsorbed deuterium and the reduction of palladium from solution. These interactions are discussed in terms of the palladium + electrolyte interphase.