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[Display omitted] • Ni/Ce x Zr 1− x O 2 catalysts for ethanol steam reforming are prepared and tested. • Oxygen vacancy over Ce x Zr 1−x O 2 depends on lattice structures and Ni–CZ interaction. • The surface oxygen vacancy generates the active oxygen species for SR and WGS. • The abundant bulk oxygen vacancy enhances the H 2 production rate at 750 °C. • The lack of oxygen vacancies limits the oxygen vacancy–involving reaction routes. Ni/Ce x Zr 1−x O 2 catalysts were synthesized with two types of Ce x Zr 1−x O 2 solid solutions to understand the role of oxygen vacancy in ethanol steam reforming (ESR). The cubic Ce 0.93 Zr 0.07 O 2 (CZ) contained more oxygen vacancies on the surface and in the lattice structure than the tetragonal Ce 0.07 Zr 0.93 O 2 (ZC). Even though the Ni-Ce x Zr 1−x O 2 interaction basically increased the surface oxygen vacancy (OV S) on the interfacial region, it also induced different bulk oxygen vacancy (OV b) behavior: no OV b over Ni/ZC and no change over Ni/CZ. In the ESR, active oxygen species (O x) generated on the OV S simultaneously involved SR, and water gas shift reaction (WGS) to produce H 2. Zero OV b over Ni/ZC influenced the O x involving–reaction routes, resulting in the limitation of WGS and the high TOF values at 750 °C. Furthermore, since the OV b played a significant role as the O x carrier, Ni/CZ showed the highest H 2 production rate in the ESR at 750 °C. [ABSTRACT FROM AUTHOR]