To find more effective microflora for the pretreatment and bioconversion of coal to methane, the combined effects of exogenous aerobic and anaerobic bacteria were studied. The changes in ultimate composition, chemical bonds, intermediates, and coal surface morphology were also investigated via the ultimate analyses, including infrared spectroscopy (IR), gas chromatography-mass spectrometry and scanning electron microscopy (SEM). Aerobic activated sludge samples and biogas slurry were used as aerobic and anaerobic bacterial sources, respectively, after acclimatization. It was found that the community composition and the relative abundances of the anaerobic microflora at the level of phylum and class were very similar to those in the formation water as recently reported. The enriched anaerobic microflora efficiently converted coal to methane with a methane yield of 77.68 mu mol/g. An aerobic pretreatment preceding anaerobic methane production tripled the methane yield (222.50 mu mol/g). The cessation of methane production during the bioconversion process did not occur because the substrates available to the microflora were depleted, but rather because of the generation of some inhibitors. Variations in the compositions of intermediates agreed well with the three stages of methane generation, indicating that different intermediates played the dominant role in each stage. Compared to the raw coal, hydrogen content decreased 11.08% after anaerobic gas production, indicating that at least part of the hydrogen in methane was derived from coal. Changes in the IR spectra of coal suggested that coal geopolymers were partially depolymerized. SEM results indicated that aerobic and anaerobic microflora could both grow on the coal surface, causing changes in the coal surface morphology. (C) 2017 Elsevier B.V. All rights reserved.