This study aimed to conduct a comparative analysis of the gas compositions at two municipal waste landfill sites over the period 2018–2023. The novelty of the study was the comparative analysis of landfill gas dynamics at municipal waste landfill sites with different levels of organization and stages of operational and post-operational. One site was unorganised, and the other one was organised. The unorganised site was used for approximately 10 years longer than the organised site, and over 15 years had passed since the end of waste disposal there. The landfill gas study involved measurements of CH₄, O₂, and CO₂, as well as the temperature and gas flow, from two landfill sites, primarily in H1 and H2. The unorganised site, which was still in the non-operational phase, exhibited an increase in oxygen (O2) concentration, which peaked at almost 18%. By contrast, the methane (CH4) concentration decreased to below 5%, indicating limited anaerobic conditions. On the other hand, high mean CH4 content (~65%), temperature (~17 °C) and CH4/CO2 ratio (2.7) were observed at the organised landfill site, along with a negative correlation between CH4 and O2, confirming the dominance of methanogenic processes. The simultaneous increase in temperature, CH4 and O2 in the second half of the study period might depend on changing environmental conditions. The moderate correlation between CH4 and CO2 at both landfill sites reflects a similar trend in their biochemical processes, despite differences related to their operational and post-operational periods.