The tetragonal ternary intermetallics RE₆Fe₁₃X (RE = Pr, Nd; X=Ag, Au, Si, Ge, Sn, Pb) , which all crystallize in the Nd₆Fe₁₃Si structure, are found to absorb approximately 12 - 18 hydrogen atoms/formula unit in a largely irreversible reaction at 390 K under a gas pressure of 1 bar. The unit cell volume expands by 10 - 15% without any change in the crystal symmetry and structure type, corresponding to an increase of volume per hydrogen atom of  ∼ 2.9x10⁻³ nm³. Lattice expansion is highly anisotropic, being an order of magnitude greater along c than along a. The anisotropic lattice expansion and hydrogenation characteristics are explained in a model where the hydrogen atoms enter the sim 0.7 nm rare-earth slabs in an amount controlled by the element X. The sim 0.5 nm iron slabs expand little. Magnetization and Moessbauer measurements on the parent compounds show them to be essentially antiferromagnetic with a net moment of 0 - 1μ_B/ formula unit but on hydrogenation there is ferromagnetic behaviour with a moment of 23 - 27 μ_B/formula unit at room temperature.