Some newly isolated microorganisms could solubilize different kinds of Spanish coals. Certain fungi and bacteria could solubilize lignite when growing in a mineral medium. However, to solubilize higher rank coals (hard coal and subbituminous coal) microorganisms require a complete medium for growth. In order to get some insight into the mechanisms of biosolubilization, cell-free culture filtrates of fungi were tested on their capacity to solubilize different Spanish coals. Two kinds of cell-free culture filtrates were used: a non-coal induced cell-free culture filtrate (NCI) and a coal induced cell-free culture filtrate (CI). Either of them was able to solubilize hard coal, subbituminous coal and most efficiently lignite. In most of the culture filtrates the measured pH values were approximately neutral, even acidic in some cases. The agent(s) for coal solubilization that was present in NCI was heat stable (100 degrees C) but the active substance(s) in CI was thermosensitive. Microorganisms were observed by electron microscopy to grow on the coal surface and cover it. A fibrilar extracellular polymer which could facilitate not only the adhesion to the coal particle but microbial attack too, was produced by the microorganisms.