<p>Undissolved organic matter (humin-HM) derived from composting of organic solid waste has received significant attention. Several studies have reported that manganese oxide could promote HM formation during the composting process. Given the importance of iron oxide in natural environments, it is worthwhile to investigate whether iron oxide could also promote HM formation during composting and compare the promoting pathway to that of manganese oxide. This study investigated the effects of iron (hematite, magnetite, ferrihydrite) and manganese (cryptomelane) minerals on HM formation during composting. Results showed that HM contents at maturity phase were 24%, 17%, 11% and 6% higher than the control when hematite, magnetite, ferrihydrite and cryptomelane were added, respectively. This suggested that iron minerals were more favorable for enhancing HM formation than the manganese mineral. Excitation-emission matrices (EEM) and two-dimensional correlation analysis of fourier transform infrared spectra (2D-FTIR-COS) of HM during composting indicated that iron minerals were more conducive to promoting the conversion of simple protein-like components to recalcitrant humic-like components in the HM structure. They also promoted the formation of HM’s core carbon skeleton (aromatic-like substances) and HM precursors than the manganese minerals. Different types of iron and manganese oxides during composting were further extracted by sequential extraction. The variations in different types of iron and manganese oxides during composting revealed that free iron oxide and carbonate manganese were strongly associated with HM formation when iron and manganese minerals were added. The findings of this study will provide a better understanding of the fate and transformation of HM in iron and manganese minerals-amended composting and further help us optimize HM products in composting.</p>