A fundamental study of CO2/CH4 plasma is performed in a glow discharge at a few Torr. Experimental and numerical results are compared to identify the main reaction pathways. OES-based techniques and FTIR (Fourier Transform Infrared) spectroscopy are used to determine molecules densities and gas temperature. Several conditions of pressure, initial mixture and residence time are measured. The main dissociation products are found to be CO and H2. The LoKI simulation tool was used to build a simplified kinetic scheme to limit the uncertainties on rate coefficients, but sufficient to reproduce the experimental data. To this aim, only molecules containing at most one carbon atom are considered based on the experimental observations. Obtaining a good match between the experimental data and the simulation requires the inclusion of reactions involving the excited state O(1D). The key role of CH3 radical is also emphasized. The good match obtained between the experiment and the simulation allows to draw the main reaction pathways of the low-pressure CO2-CH4 plasmas, in particular to identify the main back reaction mechanisms for CO2. The role of CH2O and H2O in the gas phase is also discussed in depth as they appear to play an important role on catalytic surface studied in the part II of this study