ABSTRACT ABSTRACT: The mechanical properties of high porosity chalks are strongly dependent on the type of fluids in the pores. Especially there is a strong water weakening effect. In the search for the basic mechanisms behind this water weakening effect, tensile tests were thought to be of special interest, as such tests perhaps most directly reflect the bonding properties of the material. Brazilian and direct tensile tests lead to the conclusion that the intrinsic tensile strength of high porosity chalk is best predicted from the compressional yield criterion (Coulomb). The effect of different saturating fluids are clearly reflected in the tensile strength. Therefore the use of such tests offers a convenient approach to study chalk-fluid interactions.