GORE-TEX material, commonly used in radomes, is known to be transparent at microwave bands. In ALMA a thin GORE-TEX membrane will cover the aperture through which the RF beam enters the cabin at the primary vertex hole. Slabs of GORE-TEX are also generally employed in windows for intermediate temperature shields inside the cryostat. The purpose of these windows ideally is to allow the beam to pass through them without introducing any alteration to the beam properties. The main concern is RF loss, but also cross-polarization efficiency degradation. This paper will concentrate on the results we have obtained for ALMA band 9 (602-720 GHz), in relation of loss of cross-polarization efficiency of a linearly polarized beam passing through a GORE-TEX slab, depending on its orientation relative to the direction of polarization of the beam. In order to spot anisotropic behavior of the material under test, an ad hoc measurements set-up has been used. Systematic measurements of cross-polarization properties for different thicknesses of GORE-TEX slabs were undertaken. Cross-polarization information is given in relation of the relative angle between the incident beam polarization and the material under test.