Simple esterification and etherification reactions were applied to steam-exploded Flax (Linum usitatissimum) with the aim of changing the surface properties through modification of fiber surface chemistry. Native and chemically modified cellulose fibers were characterized in terms of thermal stability, surface chemistry, morphology, and crystal structure. Independent of the substituent nature, chemically modified fibers exhibited a thermal stability comparable to that of native cellulose. Introduction of the desired chemical groups at the fiber surface was demonstrated by TOF-SIMS analysis, whereas FTIR showed that the substitution reaction involved only a small fraction of the cellulose hydroxyls. No change of the native crystalline structure of cellulose fibers was caused by chemical modification, except in the case where ether substitution was carried out in water-isopropanol medium. Cellulose fibers with unchanged structure and morphology and carrying at the surface the desired chemical groups were obtained for reinforcing applications in polymer composites. (C) 2002 John Wiley & Sons, Inc.