Hydrogen absorption to give the dihydrides MH
2+1
containing interstitial hydrogen H
i
has been studied for the metals Gd, Dy, Er, Yb and Lu in the form of films deposited in ultra-high vacuum on glass. Film areas were determined by Kr adsorption, and hydrogen content, in particular interstitial hydrogen H
i
, characterized by gas uptake, temperature programmed desorption, electrical conductivity and work function measurements by the diode method. The catalytic activity of the dihydride films for the H
2
+ D
2
→ 2HD reaction was studied at a pressure of 1.1 Torr over 175-579 K, and at 273 K over 0.19-6.2 Torr. Arrhenius plots for the rate constant show a low temperature low activation energy region changing over at a temperature
T
c
to a higher temperature higher activation energy régime, with
T
c
on average for the five metals about 50 K below the temperature
T
max
at which the interstitial hydrogen H
i
has disappeared. The suggested mechanisms are
T < T
c
: D
2
+ H
i
□
s
→ (D
2
H
i
)□
s
→ □
s
D
i
+ HD, (1)
T > T
c
: D
2
+ H
2
+ 4□
s
→ 2(D
i
□
s
) (H
i
□
s
) → 4□
s
+ 2HD, (2) where H
i
□
s
, D
i
□
s
, denotes a hydrogen, deuterium, atom held on a surface octahedral site in the f. c. c. metal sublattice. These mechanisms agree with the observed approximate first-order pressure dependency down to 77 K. The rate constants at both 273 K (under
T
c
) and 573 K (over
T
c
) decrease over Gd, Dy, Er, to Yb, and rise again to Lu, and this is discussed in terms of the metal-hydrogen, H
i
□
s
or D
i
□
s
bond strength.