Mixed proton and electron conductor ceramic composites are among the most promising materials for hydrogen separation membrane technology especially if designed in an asymmetrical configuration (thin membrane supported onto a thicker porous substrate). However a precise processing optimization is needed to effectively obtain planar and crack free asymmetrical membranes with suitable microstructure and composition without affecting their hydrogen separation efficiency. This work highlights for the first time the most critical issues linked to the tape casting process used to obtain BaCe0.65Zr0.20Y0.15O3-delta - Ce0.8Gd0.2O2-delta (BCZY-GDC) asymmetrical membranes for H-2 separation. The critical role of the co-firing process, sintering aid and atmosphere was critically investigated. The optimization of the production strategy allowed to obtain asymmetric membranes constituted by a dense 20 mu m-thick ceramic-ceramic composite layer supported by a porous (36%) 750 mu m-thick BCZY-GDC substrate. The asymmetric membranes here reported showed H2 fluxes (0.47 mL min(-1) cm(-2) at 750 degrees C) among the highest obtained for an all-ceramic membrane. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.