This guideline deals with the determination of the composition of the gas phase, at monitoring points specially installed for this purpose both within the landfill body and in the subsoil of the immediate surroundings. The compounds of interest are the main landfill gas components, methane (CH4), carbon dioxide (CO2), oxygen (O2) and nitrogen (N2), the concentrations of which are used as the basis for the assessment of potential landfill gas impacts on local environment and health. This method is also appropriate for the determination of trace components which may be present in landfill gas.