The recently developed solid-state reactive sintering (SSRS) method significantly simplifies the fabrication process for proton conducting ceramics by combining phase formation, densification, and grain growth into a single high-temperature sintering step. The fabrication simplicity provided by SSRS greatly enhances the potential for deployment of proton conducting ceramics in a number of electrochemical devices. Nevertheless, the mechanisms behind SSRS are still poorly understood. In this report, the SSRS mechanism is clarified through a systematic study of the effect of a suite of metal oxide sintering additives on the phase formation and densification of prototypical proton conducting ceramics BaCe0.6Zr0.3Y0.1O3-delta (BCZY63), BaCe0.8Y0.2O3-delta (BCY20), BaZr0.8Y0.2O3-delta (BZY20), and BaZr0.9Y0.1O3-delta (BZY10). Sintering additives with metal ions having a stable oxidation state of + 2 and an ionic radius similar to Zr4+ (which easily occupy the Zr4+ site of the BaZrO3 perovskite structure, resulting in the formation of large amounts of defects) produce the best sinterability. Sintering additives with metal ions having multiple stable oxidation states (2 +, 3+, 4+ etc.) and ionic radii near to Zr4+ (which also readily occupy the Zr4+ sites of BaZrO3 perovskite structure, but form fewer amounts of defects) can result in partial sintering by the formation of mechanically stable highly-porous microstructures with limited grain sizes. Sintering additives with metal ions having stable oxidation states + or ionic radii far from Zr4+ are unlikely to form a solid solution with BaZrO3 and yield no effect on sintering behavior (virtually identical to the control sample without any additives). These findings provide a useful guide for the selection of sintering additives to engineer optimal microstructures in proton conducting ceramics and may have potential consequences in other ceramic systems as well. (C) 2013 Elsevier B.V. All rights reserved.