Experiments have been performed in pressurized co-electrolysis mode at 800 degrees C on a typical Ni-YSZ//YSZ//CGO-LSCF cell. The polarization curves and the composition of the produced syngas have been measured at 1 bar and 10 bar. It has been found that the cell performances are improved under pressure at 1.3 V. The gas analyses have revealed that the methane formation is only activated under polarization and pressure. These experimental results have been used to validate a model which encompasses a chemical and electrochemical description of the co-electrolyser combined with a mass transport module. It has been found that the model is able to predict accurately the polarization curves as well as the syngas compositions at the cell outlet. Once validated, the model has been used to analyze the operating mechanisms in pressurized co-electrolysis. The impact of pressure on the mass transfer, the electrochemical and chemical reactions has been discussed. The close interaction between the electrochemical and chemical reactions for the internal production of CH4 has been specifically highlighted. Finally, operating maps have been computed at 10 bar from 700 degrees C to 800 degrees C. These simulations have shown that formation of CH4 in the co-electrolyser remains limited at 700 degrees C. (C) 2017 Elsevier Ltd. All rights reserved.