Characterising and determining the odorous charge of indoor air through Odour Units (OU) is an advantageous approach to evaluate indoor air quality and discomfort inside municipal solid waste facilities. The assessment of the OU can be done through the determination of Volatile Organic Compounds (VOC) concentrations and their odour thresholds. The aim of the study was to evaluate the differences in the odorous charge in the organic matter pit of a mechanical-biological waste treatment plant with a processing capacity of 287,500 tons year -1 . The sampling was carried out during the months of September 2012 (original situation) and October 2012 (after emptying the organic matter pit drain pipe). 150 chemical compounds were determined qualitatively in the studied location, from which 102 were quantified due to their odorous characteristics as well as their potentiality of having negative health effects. The results obtained demonstrated that after a maintenance cleaning operation such as draining the organic matter pit pipe, the odorous charge inside the facility can be diminished in a great way, up to a 95%