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The effect of adding small amounts of rare earth elements to palladium is to strengthen the palladium. These strenghening effects are discussed here, based on known phase diagrams of palladium-rare earths, palladium-rare earth alloying behaviour and atomic (or ionic) size effects. The solid solubilities of the rare earths in palladium, transition temperatures of various intermediate phases and eutectic temperature in these systems are influenced by the ionic (or atomic) size of the rare earth elements. A parameter, H_s, the product of the relative difference in atomic weights and the relative difference in atomic radii, between a rare earth and palladium is used to examine the solid solution strengthening effects caused by dilute rare earths. The alloying behaviours of palladium with the rare earths are very analogous, and could perhaps be used to predict alloying behaviour in some unexamined palladium systems.