Effect of the structure on desorption of water in polyamide 6 is studied by differential scanning calorimetry and thermogravimetry. Oriented and unoriented polyamide 6 fibers annealed in water at 100 degrees C for different times (2 s up to 1 h) are characterized by their structure and transport properties. It is found that water treatment leads to substantial change in the polymer structure (including phase transition even at room temperature). Activation energy and enthalpy of water desorption are determined, and their high values (E-a = 60-130 kJ/ mol; Delta H-d = 950-1360 J/g) are attributed to the formation of strong sorbent-sorbat bonds. Both parameters are influensed substantially by the polymer structure (degree of crystallinity and orientation). Besides, it is found that the temperature of water desorption (T-d) is also very sensitive to the structure. The T-d-values increase sharply with rise of degree of crystallinity, being always considerably lower for oriented fibers. This quite unexpected result is also confirmed and further interpreted in the next parts of the article. (c) 2007 Wiley Periodicals, Inc.