The biosolubilization of lignite is a promising technology for converting these low rank coals into value-added products, such as humic acid (HA), and the development of this technology has received increasing attention. In this study, Bacillus sp. Y7 was isolated from weathered lignite minerals and was shown to have effective solubilization activity on untreated Chinese lignite. More than 36.77% of the untreated Chinese lignite could be solubilized in 12 days. The lignite solubilization ability of Bacillus sp. Y7 correlated with an increase in pH, and an analysis of the extracellular substances produced by Y7 suggested that thermostable, extracellular alkaline materials in the culture were the principal factors responsible for lignite solubilization. The structure of these materials was protein-like, and nu(N-H), nu(C=O) and delta(N-H) were the main functional groups related to lignite solubilization. Micro-FTIR, NMR and elemental analyses were performed to characterize the solubilization residues and bacterial-transformed lignite humic acid (bHA) produced. Data analysis led to the conclusion that Bacillus sp. Y7 could depolymerize the non-hydrocarbon component of lignite and form bHA. The structure of bHA was similar to that of HA extracted by chemical processes from lignite but had higher N/O and H/C atomic ratios than HA. (C) 2012 Elsevier Ltd. All rights reserved.