A comparison of the plasticizing efficiency of a series of molecules introduced in dry and hydrated Kraft lignin demonstrated that the plasticization mechanisms are different before and after sorption site saturation. Lignins were plasticized in two steps and glass transition decrease was determined from DSC measurements. "Specific sorption plasticizers," such as water, are very efficient in low content because of their functionality and accessibility to sorption sites. The plasticizing efficiency does not depend on the solubility parameter, and is well quantified, considering the molar concentration of donor/acceptor functions. After saturation of sorption sites (in wet conditions), the plasticizing efficiency depends on the solubility parameter, which should be close to that of the polymer matrix. (c) 2006 Wiley Periodicals, Inc.