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X-ray diffraction, electrical resistivity, magnetic susceptibility, and specific heat measurements on Ce 1Àx Yb x CoIn 5 (0 x 1) reveal that many of the characteristic features of the x ¼ 0 correlated electron state are stable for x 0:775 and that phase separation occurs for x > 0:775. The stability of the correlated electron state is apparently due to cooperative behavior of the Ce and Yb ions, involving their unstable valences. Low-temperature non-Fermi liquid behavior is observed and varies with x, even though there is no readily identifiable quantum critical point. The superconducting critical temperature T c decreases linearly with x towards 0 K as x ! 1, in contrast with other HF superconductors where T c scales with T coh .