Reliable extrapolation of flow laws for geological materials from laboratory to natural conditions depends upon a sound micromechanical interpretation of the values of the material parameters in those flow laws. In practice, such interpretations require the experimentally determined values of the material parameters to be close to those applying during steady state deformation and/or at given material structure (mechanical state). Using an especially detailed mechanical data set for Carrara marble and a description of flow properties incorporating an explicit structure dependence, the variation of the constant strain and constant structure values of the material parameters in the exponential and power law creep equations is determined as a function of strain. The large variation that is observed underscores the need both for the acquisition of mechanical data to large strains and for further work investigating the structure dependence of flow properties, if flow law extrapolation is to be performed with confidence. ¿ 2001 American Geophysical Union