We conducted this research in order to evaluate the effect of adding foils produced from polyethylene and maize starch on the quality of composts by means of a quantitative and qualitative analysis of humic compounds. Plant waste to which an 8% addition of foil was introduced (three foils with a different share of polyethylene and maize starch were examined) was the subject of the research. Five variants of compost were subjected to analysis: C1 – biomass without the addition of foil (control), C2 – biomass + foil 1, C3 – biomass + foil 2, C4 – biomass + foil 3, and C5 – biomass + foil 3 + microbiological inoculum. Composts that contained foil but not the inoculum were characterized by a higher total carbon content than the control compost; the addition of inoculum caused a decrease in total carbon content. The compost did not differ significantly in the humic acid carbon content, but a diversity in the fulvic acid carbon content was found. The ratio of humic acid to fulvic acid carbon content was most favorable in the (C2) compost. The addition of foil led to a slowdown of biomass humification. Composts with the addition of foil contained more non-hydrolyzing carbon than the control.