We theoretically examine four hypothetical ternary lanthanide hydrides, CsLnIIH3 and Cs2LnIIH4, where LnII = Yb, Tm. We optimize their crystal unit cells in the BaTiO3 and K2NiF4 structures, respectively, and compute their electronic band structures. Our calculations indicate that the novel hydrides should be unstable with respect to decomposition into binaries (CsH and LnIIH2); ternaries would form only under elevated pressure (>7–22 GPa). We predict that significant perturbation of the electronic and magnetic properties of CsYbIIH3 and Cs2YbIIH4 will take place via a progressive exchange of f14 YbII for f13 TmII, while promoting magnetic ordering and valence fluctuations. Analysis of the phonon dispersion for these hydrides suggests that metallic forms of doped CsLnIIH3 and Cs2LnIIH4 would exhibit substantially high Debye temperatures of ~1800 K. This is likely to prompt moderate-TC superconductivity in these as yet unknown materials.