Mounting evidence suggests that subcritical crack growth is an important mechanism for the development of natural fractures. Numerical simulations of fracture patterns are sensitive to the subcritical crack growth index, the exponent used to describe the power law dependence of crack velocity on stress intensity. Few measured values of subcritical indices in sedimentary rocks have been published, however, and no systematic studies of diagenetic controls of subcritical crack index behavior are available. Subcritical crack index measurements are especially difficult in weak, porous rock, primarily because non-elastic material response can obscure fracture-related behavior. We have carried out numerous subcritical crack index measurements with the widely used dual torsion beam test configuration and developed a modified test methodology for subcritical crack index measurements on porous rocks. The principal element of the methodology is a specimen pre-loading procedure to minimize transient material response. Measurement reliability is documented in the study by documentation of reproducibility in three sets of sandstone and carbonate specimens. ¿ 2001 American Geophysical Union