During litho-stratigraphic studies of deposits in small mires in the upper basin of the Oder River, mid-Holocene hiatuses identified by the presence of horizons of strongly decomposed peat (HSDP), were identified at four sites. Radiocarbon dating results indicate long (103 years) breaks in peat accumulation and the dates obtained from the underlying layers of deposit are inconsistent with the pollen record. In order to explain these discrepancies, an attempt was made to recreate the conditions of HSDP formation under the influence of endo- and exogenous factors determining peatland productivity. In the light of macrofossil analysis, the development of HSDP during the Atlantic and Subboreal periods, when the humidity of habitats decreased, occurred at an advanced stage of development of swamps with