A waterlogged wooden <i>Oberländer</i>, a medieval river craft from the Rhine was salvaged in 1973. It was built of very wide boards, up to 12 cm thick, of European oak. Individual boards comprised wood in different states of degradation. A two-step polyethylene glycol (PEG) treatment was thus applied for the dimensional stabilization of the timbers. Progress of the impregnation with 43% PEG 200 and 70% PEG 3000 was monitored by high performance liquid chromatography (HPLC) of extracts taken from fine wood cores. These were taken at intervals with an increment borer and segmented after removal. Total stabilization was achieved but the PEG analysis showed the necessity of inserting an extraction step between the two PEG impregnations to increase surface hardness.