Lithium-oxygen batteries differ from other classical battery technologies because they involve reactions with gaseous species, for which valuable information can be gleaned from quantitative and qualitative investigations. In this study, we report a modified design for an electrochemical test cell that integrates a pressure sensor, thereby enabling accurate in operando monitoring of pressure changes during charge/discharge with high reproducibility and sensitivity and without disturbing the cell system. Its use is demonstrated by quantifying the parasitic reactions in Li-O-2 cells based on a carbon cathode, as a function of the various electrolytes frequently encountered in the literature, such as LiTFSI in DME, DEGDME and TEGDME, LiNO3 in DMA, and LiClO4 in DMSO. Through this comparative study, we are able to observe the phenomena currently limiting the performances of Li-O-2 batteries. Moreover, the long-term cycling behavior of the cells shows a similarity for all the electrolyte systems investigated - a formatting process occurring during the first cycles - which illustrates the importance of the set-up reported here, enabling easy gas monitoring over unlimited number of cycles. This technology is directly transferable to the study of every material whose electrochemical behavior enlists gas uptake and release such as Li-rich layered compounds, organic electrode materials and other metal-air batteries. (C) 2016 The Electrochemical Society. All rights reserved.