Humic-reducing microorganisms (HRMs) can utilize humic substances as electron shuttles to transform contaminants in different environments. Composts as organic agents contain large amount of humic acids (HA), and the formation of HA can support the growth of HRMs during composting. However, the effects of HRMs from different compostable environments on pentachlorophenol (PCP) dechlorination via corresponding composting-formed HA remain unknown. In this study, HA extracted from protein-, lignocellulose-, and lignin-rich composts, were examined in the corresponding HRM-mediated reductive dechlorination of PCP. Results showed that PCP could be dechlorinated by HRMs from different composts, and the highest rates were observed in HRMs derived from protein-rich composts. The addition of HA influenced the microbial degradation efficiency but did not change the PCP dechlorination pathway. HA extracted from lignocellulose-and lignin-rich composts could act as electron shuttles to enhance HRM-mediated reductive dechlorination of PCP. Nine HRMs that significantly influenced ortho-dechlorination and trichlorophenol transformation were selected on the basis of the relationships between key HRMs and intermediates. Consequently, understanding the relationships between key HRMs and environmental factors could give insight into mechanisms of microbially reductive transformation of PCP by composts in the chlorophenol-contaminated environments.