Abstract Organic fraction (OF) from municipal solid waste (MSW) was sampled from several Spanish composting facilities just before the composting process began, considering the two groups of OF treated at the current time: OF from source sorted collection (SC) and OF from non-source sorted collection but selected in the plant by mechanical devices (MS). Moisture content (MC), pH, electric conductivity (EC), total organic matter (TOM), resistant organic matter (ROM), organic nitrogen (org-N), resistant organic nitrogen (nh-N), and soluble inorganic nitrogen (NH 4 +-N) were analysed from both samples. These parameters are significant in the composting process and have environmental implications. Most of the Spanish composting facilities treat MS from the MSW collected in mass. This study was carried out to assess and underline the variation in the composition of OF depending on the sorting system. Significant differences were detected between the two groups. SC showed higher values for MC, TOM, org-N than MS which resulted in the highest pH and EC values. No differences were detected for ROM and nh-N, indicating a similar degradable potential. Based on the results of this study, it is recommended that the composting process be carried out in a way that reflects the category of OF treated as well as considering the addition of yard trimmings to avoid leachates formation, balance the C/N ratio and improve aeration.