Drift wood (or in-stream large wood, LW) plays an important role in river ecosystems by influencing hydrodynamics and morphology. Wood density, therefore, buoyancy, is the main factor conditioning initial motion of wood in rivers. The better understanding of wood density and the dry-wet process and decay, may be useful to improve the knowledge of wood dynamics in rivers. We analyze wood pieces retained in a dam, in the French Rhone, and a set of freshly cut riparian trees. Different protocols were set to measure density and buoyancy of these two series of wood and to test effects of drying and wetting, species and wood decay stages. Preliminary results after seven months of experiments show different behaviors in density and buoyancy depending on species. Light wood (360-500 kg · m3) is likely to show a buoyancy rate about 52% (half of the log emerged), medium density wood (500-700 kg · mr 3) about 39%, 21% for dense wood (700-900 kg-mr3) and 12% for very dense wood (>900 kg-mr3). We observed a significant negative correlation between wood density and buoyancy and proposed a model to predict wood buoyancy. The results from this work will help to understand the evolution of buoyancy through time and estimate local conditions of entrainment and transport of wood in rivers.