In this publication, the performance of a slurry bubble column reactor (SBCR) and a tube bundle reactor (TBR) are compared for steady-state and transient Power-to-Gas (PtG) operation. Transient PtG conditions are modeled using gas load step changes between 25 and 100% of the reactor maximum capacities in 1 s. For steady-state operation the TBR facilitates much higher gas hourly space velocities (GHSV) as compared to the SBCR. A sensitivity analysis shows that the TBR is limited by heat transfer, while the SBCR is limited by gas/liquid mass transfer. For transient PtG operation the TBR undergoes significant temperature changes within a short time resulting in out of specification product gas qualities and unacceptable temperature hot spots; the SBCR temperature shows marginal changes upon transient operating conditions, and the outlet gas composition sticks to the gas quality requirements. Finally, measures to improve the efficiency of both reactors are proposed considering dimensionless numbers. The GHSV of the SBCR can be enhanced by increasing the specific interfacial area controlling gas/liquid mass transfer, while the transient behavior of the TBR can be improved by reducing the catalyst concentration/activity or by mixing the catalyst with high heat capacity inert material in detriment of the GHSV. (C) 2019 Elsevier Ltd. All rights reserved.