Demand for natural gas is expected to increase faster than any other fossil fuel over the coming decades. Australian coal bed methane (CBM) resources are among the largest in the world and are already being utilized to meet increasing demand. The majority of methane contained within CBM producing coal beds is microbially generated and low rank coals are often associated with higher bioavailability. However, the results of previous studies are conflicting, and it is unclear how or if coal rank has an effect on microbial community structure. Here, enrichment cultures grown on coals spanning 13 different ranks (lignite to bituminous) were characterized with 16S rRNA gene amplicon sequencing and combined with volatile fatty acid (VFA) and headspace methane measurements to understand the effect of coal rank on CBM microbial communities. Our results show that there is a significant negative correlation between final methane yield and rank, suggesting that the bioavailability of the coal organic material decreases with increasing thermal maturity. The concentration of VFAs generally increased with decreasing rank and revealed that community composition was significantly correlated with the concentration of specific VFAs (e.g. acetate). These data suggest that the observed higher methane production from lower rank coals is linked to increased concentrations of low molecular weight acids desorbing from the coal. The increase in low molecular weight VFAs was associated with the enrichment of specific taxa known to specialize in the degradation of low molecular weight acids and alcohols. (C) 2016 Elsevier B.V. All rights reserved.