Low molecular weight polyphenols and ellagitannins were analyzed by HPLC, and the molecular weight distribution of ellagitannins was calculated by GPC, in oak heartwood of Quercus robur L., Quercus petraea Liebl., Quercus pyrenaica Wild., and Quercus faginea Lam., grown in Spain, before and after 1 year of seasoning, in Bordeaux, France. During this process, the concentrations of low molecular weight polyphenols (acids and aldehydes, benzoic and cinnamic, and coumarins) increased, and those of ellagitannins (castalagin, vescalagin, and roburins A-E) decreased. A similar behavior for the A and B compounds in all species was not found. This modification in the chemical composition was similar in the four Spanish species of Quercus studied and allowed the differentiation between the unseasoned wood and the wood after the first year of seasoning.