The Ca-bentonite occurrences in Edirne-Lalapasa region of NW Turkey comprise economically important minable deposits which originate from the Eocene and Oligocene formations exposed in the northern part of the Tertiary Thrace Basin. The bentonite levels contain high amounts of dioctahedral Ca-smectite (montmorillonite) formed by devitrification of volcanic glassy fragments and also crystallization from feldspar grains. These differentiations on the formation and structure of smectite were envisaged to cause variations on parameters like zeta potential and cation exchange capacity. In this study, nine different Ca-bentonite samples with relatively high smectite contents were used in order to correlate the zeta potential values of these samples with particle size, pH, cation exchange capacity (CEC) and also smectite structural formulas and layer charges. A new methodology for determining the relationships between zeta potential of bentonite aqueous dispersions in their natural pH and Al3+ and Mg2+ content in the octahedral sheet in smectite structure is proposed. Accordingly, the zeta potential range of - 9.5 and - 17.4 mV was found to reasonably correlate with the octahedral Al3+ and Mg2+. The negative value of zeta potential of the Ca-bentonites studied increases with increasing octahedral Al3+ and decreases with octahedral Mg2+ in the smectite structure.