The organic content of Garumnian coals in the central Pyrenees of Spain is heterogeneous and is formed of plant fragments, with the fossil remains of botanic structures and secondary products resulting from thermal and biochemical transformation of macerals. The maceral composition of the three areas studied, Berga, Tremp and Ager, is similar. The Berga and Tremp zones include an upward decrease in organic materials and the top sequence is significantly richer in inorganic sediments. In contrast, in the Ager zone only one level with organic matter is present. The organic matter of the Tremp Formation is humic and five resinite groups have also been characterized. The mineral phases determined, mainly carbonates, appear closely associated with macerals, infilling cell lumens or replacing cell walls, and in layers within the organic matter. The maturity and source rock potential of the organic rich beds were investigated using reflected light and fluorescence microscopy. The results are compared with other maturity parameters (Rock-Eval, calorific value and diagenetic mineral matter). The vitrinite reflectance (R(o) 0.35-0.45) and the maximum temperature (430-degrees-C) are low and homogeneous in the different areas. These values indicate that the rank of the samples corresponds to an immature diagenetic state close to the lower oil window and allows classification of this coal as subbituminous. The relationship between inorganic and organic phases indicates low-grade transformation as a consequence of a low lithostatic pressure. The macerals indicate that the coal-forming peat was deposited in a predominantly forest-moor swamp environment. The liptinite assemblage suggests a subtropical and damp climate during peat deposition. The amount of organic matter decreases upwards within the sedimentary sequences analyzed. Palaeoenvironments were similar in the three areas, which could originally have been part of the same basin.