Mixtures of 6LiBH4–RECl3–3LiH (RE = La, Er) have been produced by mechanochemical milling and their structure, thermal decomposition and reversibility have been studied. Hydrogen desorption starts around 300 °C in both composites. Heating to 400 °C yields LaB6, ErB4 and REH2+δ as major decomposition products. LiBH4 is destabilized by REH2+δ formed through decomposition of the parent borohydrides LiLa(BH4)3Cl and Er(BH4)3, respectively, and its hydrogen release temperature is reduced by 100 °C as compared to pure ball-milled LiBH4. The lanthanum-containing composite releases 4.2 wt.% H between 300 and 350 °C and shows a limited reversibility of ∼20% (340 °C, 10 MPa) probably due to hydrogen uptake by some amorphous boron-containing phases. For 6LiBH4–ErCl3–3LiH about 3 wt.% H is evolved up to 400 °C. Desorption against 0.5 MPa backpressure results in an increased reversibility (∼80%) as compared to vacuum (∼66%). Rehydrogenation (340 °C, 10 MPa) shows the formation of ErH3 and LiBH4 at drastically reduced conditions compared to pure LiBH4 (>400 °C, >10 MPa).