Humification during co-composting of food waste, sawdust and Chinese medicinal herbal residues (CMHRs) was investigated to reveal its correlation with compost maturity. Food waste, sawdust and CMHRs were mixed at 5: 5: 1 and 1: 1: 1 (dry weight basis) while food waste: sawdust at 1: 1 (dry wt. basis) served as control. Lime at 2.25% was added to all the treatments to alleviate low pH, and composted for 56 days. Humic acid/fulvic acid (HA/FA) ratio increased to 0.5, 2.0 and 3.6 in the control and treatment at 5: 5: 1, and 1: 1: 1 mixing ratio, respectively at the end of composting. The decrease in aliphatic organics in HA demonstrated the degradation of the readily available organics, while an increase in aromatic functional groups indicated the maturity of compost. Disappearance of hemicellulose and weak intensity of lignin in the CMHRs treatments indicated that the lignin provided the nucleus for HA formation; and the CMHRs accelerated the compost maturity. (C) 2014 Elsevier Ltd. All rights reserved.