The saturation of polydienes with deuterium produces labeled polyolefins that, when studied with the identical hydrogenated polydienes, have proven quite useful for the understanding of many of the basic properties of these materials. However, until now the only way to determine the amount of deuterium substitution on the labeled molecules has been to compare the densities of the hydrogenated and deuterated polymers, which gives no information on how the deuterium is distributed along the chains. The information is important, since it is clear from the density results that with some saturation catalysts (Pd) there must be some H-D exchange since there are more than two deuterium atoms per diene monomer on the labeled species, while for others (Wilkinson's) there appears to be no exchange. In this work, pyrolysis-photionization mass spectrometry has been used to examine the structure of the hydrogenated and deuterated polydienes in more detail. The deuterium distributions were determined using a modified Bernoullian model that takes into account the first two D's that were added by the saturation event. The most revealing data come from the tetramer C4H8-xDx, which confirm the average level of deuteration as measured by density. For polymers saturated with Pd catalysts, the pyrolysis-photionization mass spectra show that the deuterium was randomly distributed by the H-D exchange. Perdeuteriopolyisobutylene was also examined, as was its blend with hydrogenous polyisobutylene, which helped to confirm that exchange had occurred during the saturation of the polydienes using the catalyst rather than during the measurement step. The data for the polydiene deuterated using Wilkinson's catalyst confirmed that there was little, if any, exchange.