This paper presents the water vapour sorption behaviour of degraded archaeological oak (Quercus robur L.) and the influence of methyltrimethoxysilane treatment on hygroscopicity. Wood samples (archaeological and undegraded recent oak) were treated with methyltrimethoxysilane using an oscillating pressure method. Moisture properties of the samples were determined using a dynamic vapour sorption system, and the surface area and porosity of treated and untreated waterlogged wood, previously dried using different methods, were characterised using a nitrogen sorption method. It was found that the silane modification resulted in a decrease in the equilibrium moisture content of archaeological oak samples from 23.7 to 19.4% for heartwood and from 23.3 to 10.0% for sapwood, respectively. After correction for silane content, however, the maximum equilibrium moisture content of the treated samples was 23.6% for heartwood and 21% for sapwood, which points rather at a bulking mechanism than chemical modification by silane. The results of the surface area and porosity measurements indicate that methyltrimethoxysilane is deposited in the cell wall and thus helps to preserve the microstructure of archaeological waterlogged wood.