In the present study, the synthesis of two different LiBH4-Y(BH4)(3) and LiBH4-YH3 composites was performed by mechanochemical processing of the 4LiBH(4)-YCl3 mixture and as-milled 4LiBH(4)-YCl3 plus 3LiH. It was found that Y(BH4)(3) and YH3 formed in situ during milling are effective to promote LiBH4 destabilization but differ substantially from each other in terms of the dehydrogenation pathway. During LiBH4-Y(BH4)(3) dehydriding, Y(BH4)3 decomposes first generating in situ freshly YH3 and subsequently, it destabilizes LiBH4 with the formation of minor amounts of YB4. About 20% of the theoretical hydrogen storage was obtained via the rehydriding of YB4-4LiH-3LiCl at 400 degrees C and 6.5 MPa. As a novel result, a compound containing (B12H12)(2-) group was identified during dehydriding of Y(BH4)(3). In the case of 4LiBH(4)-YH3 dehydrogenation, the increase of the hydrogen back pressure favors the formation of crystalline YB4, whereas a reduction to <= 0.1 MPa induces the formation of minor amounts of Li2B12H12. Although for hydrogen pressures <= 0.1 MPa direct LiBH4 decomposition can occur, the main dehydriding pathway of 4LiBH(4)-YH3 composite yields YB4 and LiH. The nanostructured composite obtained by mechanochemical processing gives good hydrogen storage reversibility (about 80%) regardless of the hydrogen back pressure. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.