In the "Solling-Projekt" (IBP) of the "Deutsche Forschungsgemeinschaft" the decomposition of different qualities of leaves in acid soil (Solling) and in lime soil (Göttinger Wald) was studied under various conditions (October 1973-October 1974).Plastic rings were used as litter bags in the following way: on the bottom side they were covered with gauze of different mesh sizes (50, 1120, or 6000 μm), after they had been filled with leaves the upper sides were closed by a gauze with an equal mesh size (about 1200 μm). The effects of abiotic factors, microorganisms, meso- and macrofauna, phytophagous-excrements, and leaf-powder are shown by the following results: 1. In lime soil the decomposition was higher than in acid soil. 2. The decomposition effected by microorganisms alone was less than that by microorganisms and mesofauna; this, however, was less than the one by microorganisms, meso- and macrofauna. 3. Shade-leaves were decomposed more than sun-leaves and these more than leaves 1 year old. 4. In acid soil phytophagous-excrements effected a very high decomposition while in lime soil an effect was not to be found. 5. Leaf-powder effected a very high decomposition of sun-leaves and leaves 1 year old in both forests, of shade-leaves, however, in acid soil only. 6. In the Solling (acid soil) and in the Göttinger Wald (lime soil) shadeleaves from the Solling as well as from the Göttinger Wald were decomposed equally. 7. In the Solling the decomposition of leaves with phytophagous-excrements or leaf-powder compared with the decomposition in the Göttinger Wald without phytophagous-excrements or leaf-powder was nearly the same if mesofauna was allowed to enter, but the decomposition was highly significantly smaller if macrofauna was allowed to enter, which was rather scarce. 8. In the winter half-year the decomposition was smaller than in the summer half-year. The decomposition in the winter- and the summer half-year together (October 1973-April 1974 and April 1974-October 1974) was considerably higher than the decomposition of that litter which had been exposed for a whole year without interruption. 9. Some of the leaves sprayed with Toluol were partly destroyed more than those sprayed with Naphthalin.