The alloying characteristics of the rare earth elements with the transition metals undergo a radical change as the atomic number of the transition series increases - - transition elements in Groups IVa, Va, and VIa are immiscible with the rare earths, while elements of Groups VIIa, and VIIIa, VIIIb, and VIIIc form many compounds. Since this cannot be correlated with a size effect, a reasonable explanation for this behavior is a valency or electronegativity effect. Those binary systems forming compounds form 'Laves phases', which can exist in one of three related crystal structure types: MgCu2, MgZn2, or MgNi2. The specific Laves type crystal structure can be related to the average free electron concentration, a phenomenon which has been used to calculate electronic valency of the transition elements. A compilation of the known Laves-type phases occurring between rare earth elements and transition metals supports the hypothesis that the valency effect is operative. Forty-two additional rare earth-transition metal compounds previously unknown have been prepared and found to be consistent with the previously noted trend with but two exceptions. On the assumption that a critical electron/atom ratio determines which Laves-type structures are stable, the periodical grouping of the Laves-type species of the rare earth-transition metal compounds indicates a slight but regular increase in valency as the atomic number of the rare earth increases. Ternary alloys prepared between the Laves phases of different structure types substantiate the observed valency trend.