Ammonia borane (AB) containing 19.6 wt % hydrogen is among the strongest candidates for hydrogen storage on-board fuel cell vehicles, though efficient means for hydrogenating the spent material are needed. We are investigating a scheme in which spent material is digested to BX3 using common HX reagents. The hydride affinity of BX3 is a key factor in selecting HX to optimize the efficiency of AB regeneration. We have calculated hydride affinities of various BX3 compounds using a simple isodesmic reaction scheme. The results show hydride affinity depends on the heteroatom attached to boron, as well as the nature of the organic group. These calculations suggest that BX3 compounds with X = SR, SAr and OAr are among the strongest candidates for use in regeneration schemes. Results of these calculations and supporting experiments will be presented. Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.