Greenhouse gas (GHG) accounting with respect to two categories of methane recovery and oxidation activities (coal bed or coal mine methane recovery and landfill gas (LFG) recovery) within the Clean Development Mechanism (CDM) is analysed. It is found that baseline methodologies approved by the CDM Executive Board apply systematically inconsistent assumptions concerning the global warming impact of carbon dioxide emissions from the oxidation of methane. One important implication of the results is that applying the baseline methodologies approved for project activities involving LFG recovery will lead to overestimation of the net GHG abatement effect of such CDM project activities.