Isothermal calorimetric studies of the D/Pd and H/Pd systems have been carried out at high deuterium (hydrogen) loadings (i.e. [D(H)]/[Pd] > 0.9) at approximately 30-degrees-C. Under these conditions, the generation of "excess power" was observed in a series of deuterium-based experiments, but not in a hydrogen-based experiment. The results of these experiments enable several (tentative) conclusions to be reached concerning the conditions necessary for the reproducible observation of this anomalous thermal effect.