The heat of combustion of five different clays, all exchanged with different tetraalkylammonium cations, was determined. The relationship between heat of combustion and the cation exchange capacity was investigated. The heat of combustion of the tetraalkylammonium clays was determined by using bomb calorimetry. Five standard smectite clays were used; SCa-3, STx-1, SWy-2, SHCa-1, and SAz-1. Each clay was modified with eight different tetraalkylammonium salts resulting in 40 samples. Each sample was analyzed using bomb calorimetry and x-ray diffraction (XRD). X-ray diffraction provided information about the arrangement of the tetralkylammonium cations in the interlamellar region of the clay and an indication of when the charged sites become inaccessible to the cations. Bomb calorimetry proved to be a good way of determining the heat of combustion of the modified clays. As the number of methylene groups increased, the heat of combustion increased, giving linear correlation until the point when the intercalated salts become sterically hindered.