Activity, as introduced in 1953 by Skempton, was defined as the ratio of plasticity index (PI) to the clay fraction (CF) of a soil and was used by Skempton to help distinguish mineralogic differences in fine-grained soils. While activity (A) provides an approximate method of delineating fine-grained soils by mineralogy, PI is not really a fundamental soil property. Conversely, Specific Surface Area (SSA) and Cation Exchange Capacity (CEC) are more fundamental soil properties that dominate engineering behavior of fine-grained soils. SSA and CEC are “inherent” soil parameters and may be combined with the clay fraction in order to help classify mineralogic composition of fine-grained soils. Two parameters are examined: 1. Specific Surface Area Activity (S_c), which is defined as the ratio of surface area to the clay content, and 2. Cation Exchange Capacity Activity (CECA), defined as the ratio of cation exchange capacity to the clay content. These two parameters show clear groupings of low, medium and high “activity” when used in relationship with other index parameters that may help explain fundamental clay behavior. The relationships between S_c and CECA and engineering properties are presented and described. The results indicate that Specific Surface Area Activity and Cation Exchange Capacity Activity may be a more fundamental and convenient basis for describing the mineralogic composition of fine-grained soils.