Designing modern devices for bulk solids storage and handling requires an increasingly complex understanding of the handled bulk solids＇ mechanical behaviour. Models able to describe the behaviour of bulk solids can neither be formulated nor their parameters determined from common shear tests, since these do not contain sufficient information especially about the materials＇ stiffness characteristics. The paper describes the response envelope as a measure of the deformation resistance and its experimental determination from biaxial tests. It is shown that for fine limestone powder the stiffness in compression is only about 1/70 of the stiffness in expansion. The shape of the response envelope depends only on the ratio between the principal stresses at the stress state considered, whereas the size of the response envelope also depends on the stress magnitude.