A batch of very dry lithium metaphosphate glass, containing 1% potassium metaphosphate, was prepared and dissolved completely in nearly pure heavy water. From this solution, a series of glasses with varying deuterium oxide content was prepared. The internal friction and dielectric loss data were taken. The amount of dissolved deuterium oxide was determined from the weight-loss of specimens during heating in vacuum, and checked by means of tritium labeling. It is found that dissolved deuterium oxide influences the internal friction and dielectric losses, in the same was as additions of dissimilar alkali ions do (mixed alkali effect) so that one can say that deuterium behaves similarly to alkali ions. A comparison of the present data with the results of a recent study on the influence of protons on the mechanical and electric properties in an identical host glass illustrates clearly that the mixed alkali effect is not governed by mass differences of the cations.