abstract: The effect of concentration of lignite, derived from the Polish mine in Bełchatów, pretreated with 8M nitric acid, on the yield of its biosolubilization by Fusarium oxysporum 1101 strain was studied. Observed decrease in pH during lignite biosolubilization by the F. oxysporum 1101 was correlated with amounts of humic and fulvic acids released from the substrate. Conversely, in the lignite-free medium the strain synthesized alkaline compounds that caused an increase in pH. The highest biosolubilization yield was achieved when the culture medium was supplemented with 5% of  lignite (3-times higher than in the medium supplemented with 10% of coal). In this case, for the first 8 days, the products with gradually decreasing molecular masses were efficiently released (the E₄/E₆ ratio was growing) and then polymerized (the E₄/E₆ ratio was reduced), and the content of condensed aromatic rings was decreased. The FTIR analysis showed that the relative intensity of the peaks reflected the concentration of functional groups in the tested products. Only the spectrum of products released from 10% coal contained the additional peak, corresponding to -CH(Ar) groups.