The aim of the research was to assess the quality of organic matter contained in sewage sludge composting products and their co-composting with fly ash and mineral wool. The object of the research were composts produced using stabilized sewage sludge from the municipal sewage treatment plant (SS_1C) and sewage sludge with the addition of 20% (SSF_2C) and 30% (SSF_3C) of fly ash and 5% (SSW_4C) and 10% (SSW_5C) of mineral wool. Selected physicochemical properties, fractional composition of humic compounds, and the degree and rate of humification were determined in compost samples taken after 180 days of composting. The reaction of the evaluated composts was close to optimal for mature composts. Co-composting of sewage sludge with mineral wool and ash increased the sorption capacity in composts compared to SS_1C. Due to the content of available P and Mg, the discussed composts formed the SS_1C>SSF_2C and SSF_3C>SSW_4C and SSW_5C series. However, in terms of available K content: SSF_2C and SSF_3C>SSW_4C and SSW_5C>SS_1C. In the SS_1C compost the organic carbon (TOC) content was slightly higher, but no statistically significant effect of the addition of fly ash and mineral wool on the TOC content in mature composts was confirmed. The addition of ashes and mineral wool significantly increased the total nitrogen content. Due to the humification index, the composts formed the series: SSW_4C > SSW_5C > SSF_2C > SS_1C > SSF_3C. The values of the C-KH/C-KF ratio in SS_1C were typical for good quality soils, while in the remaining composts the C-KH/C-KF values were slightly lower. The degree of humification of the assessed composts was characterized by poorly humified organic materials, with the highest values of this indicator found in composts with the addition of mineral wool. The assessed quality indicators of organic matter indicate that the organic matter of composts from sewage sludge with the addition of mineral wool and 100% sludge was of the highest quality.