Abstract A limited number of South Scandinavian barrows from the Bronze Age contains several burials with well-preserved organic material including remnants of the dead. In some cases there are indications of an accumulation of the burials over some time before the barrow was erected. This raises the question of how the decomposition of the organic matter was hindered until the barrow construction. Experiments with deposition of piglets in models of Bronze Age oak log coffins with and without covering barrows were conducted, and the development of oxygen content in the coffins was monitored. The experiments resulted in different oxygen depletion patterns, which were compared with the preservation of the piglets upon excavation of the barrows. Coffins covered with a thin layer of grass sods quickly developed an anoxic environment and the decomposing processes were slowed considerably, whereas the free standing coffins remained oxygenated and the piglets decomposed almost completely within a period of three months. The results allow conclusions on the decomposing processes in the barrows and suggest that an interim cover of the burials was necessary to preserve the bodies until the barrow could be constructed.