The elastic moduli (and acoustic velocities) of shaley sandstones strongly depend on the amount of clay as well as on its position among sand grains. In many cases clay may fill pores without noticeably affecting the stiffness of the rock. Such clays are non-load-bearing. Here, the stiffness of the rock is determined primarily by the load-bearing quartz frame. We examine several data sets and show that in this case a simple unique relation exists between the acoustic velocity and the porosity of the load-bearing frame of the rock. This porosity can be calculated as the sum of the effective porosity (total porosity minus microporosity inside clay) and the volumetric fraction of clay in the rock. Also, it is the sum of the total porosity and the volumetric fraction of solid clay particles in the rock. ¿ 1999 American Geophysical Union