To circumvent the deleterious early decomposition effects of thermally unstable alkyl ammonium-treated montmorillonites when processed with high-melting-point polymers, high thermally stable salts are used as intercalant agents for sodium montmorillonite rich bentonites of Turkish origin, by an ion exchange reaction to produce organo-montmorillonites with enhanced thermal stability. The bentonite clay was purified by sedimentation, resulting in higher cation exchange capacity and thermal stability in comparison to unpurified clay, and then used in the synthesis of six thermally stable organo-montmorillonites by replacing the interlayer inorganic sodium cations with two (alkyl, aryl) phosphonium and four di-(alkyl, aryl) imidazolium surfactant cations. The interlayer spacing of the phosphonium organo-montmorillonites ranged from 1.78 nm to 2.52 nm, while the interlayer spacing of imidazolium organo-montmorillonites ranged between 1.35 nm and 1.45 nm. The effects of chemical structure (chain type), counter ion and alkyl chain length on the thermal stability of the imidazolium salts were investigated. (C) 2011 Elsevier B.V. All rights reserved.