Electrothermal propulsion higher limits of projectile velocity. It provides many advantages over conventional propulsion. As for a conventional gun, the knowledge of thermochemical data and propellant behaviour is necessary to optimize the system. The reaction products issued from the decomposition of methanol during an electrothermal process were characterized in order to understand the chemical process. Therefore, a plasma generator is specially added to a closed vessel suitable for high pressures. The combustion products of methanol were identified and quantified by the analysis of the gases with a chromatograph coupled with a mass spectrometer. The released chemical energy is calculated from the analysis results. A defined electrical energy injected into the plasma is found by which the amount of methanol is completely decomposed. This energy is estimated by an empirical method. The theoretical calculations of temperature fit well with the observed phenomena. Results indicate that the plastic materials used in the closed vessel have an important influence on the combustion. Moreover, the chemical energy released during reaction is not the decisive parameter for the optimization of the launcher＇s performances during the electrothermal process. A relation between the chemical energy released during reaction and the plasma injection energy is established for methanol.