At the Koźmin Las site, in Central Poland, in the middle section of the Warta River valley, a series of well-preserved tree trunks and in situ stumps, as well as organic deposits, have been found. The tree remains are dated back to the period between 13,000–12,600 cal BP, i.e. to the Alleröd/Younger Dryas transition. The forest consisted predominantly of pines (Pinus sylvestris L.) of a maximum age of approx. 140 years and an average age of 68 years, and the river valley floor was overgrown. The forest was destroyed ca. 12,600 cal BP by deteriorating hydrological conditions or a sudden catastrophic event. The aim of the study was to assess the degree of degradation in terms of selected macroscopic, physical and chemical properties of a subfossil pine log. On this basis, a conservation process was developed,using aqueous solutions of polyethylene glycols (PEG) with varying concentrations of low- and high-molecular polymers. Treated and dried samples were comparedin terms of their tangential and radial dimensional stability, as well as their hygroscopic properties.