Abstract: Fibers composed of a vinyl alcohol unit-containing polymer, or composite fibers, yarns or fiber products composed of the phase of the vinyl alcohol unit-containing polymer and the other fiber-forming polymer phase are acetalized with at least one of aldehyde compounds represented by the formulas,OHC--CH.sub.2 --C(R.sup.1)(R.sup.2)--COOBandOCH--C(R.sup.3)(R.sup.4)--COOBwherein B is a hydrogen atom or an alkyl group, R.sup.1, R.sup.2, R.sup.3 and R.sup.4, independently from each other, denote a hydrogen atom or an alkyl group, and at least one of R.sup.1 and R.sup.2 and at least one of R.sup.3 and R.sup.4 are alkyl groups,and a carboxyl group is then converted into a salt to form fibers, yarns or fiber products having high moisture absorption and high water absorption.