This study describes the use of cassava starch acetate (CSA) to impregnate Kraft paper and its influence on the water vapor permeability (K-w). This barrier property depends on the water vapor diffusion coefficient (D-eff) and the material hygroscopicity properties (adsorptivity, beta). The influences of D-eff and beta on the K-w value were studied. Kraft paper samples were impregnated at atmospheric pressure and under vacuum. The results showed an important decrease on the K-w values, which was associated with two factors: (a) the partial filling of the Kraft paper superficial and internal pores by the impregnated CSA, reducing the K; (b) as CSA is much less hygroscopic than paper, its adsorptivity is reduced significantly by impregnation. This paper demonstrates that CSA impregnation of Kraft paper is an interesting alternative for the improvement of the hygroscopic properties and water vapor permeability of the Kraft paper. (c) 2004 Elsevier Ltd. All rights reserved.