A calix[6]arene carboxylic acid derivative incorporating tert-octyl groups at the upper rim was found to be soluble in an aliphatic organic solvent. The extraction behavior of various nucleobases was investigated with the calix[6]arene carboxylic acid derivative. In an isooctane system, neither calix[6]arene nor D2EHPA were very effective at nucleobase extraction. However, addition of D2EHPA along with calix[6]arene drastically enhanced the efficiency of adenine and cytosine extraction. It was found that D2EHPA is a powerful synergistic reagent with calix[6]arene for adenine extraction. It was demonstrated that one nucleobases forms a stable complex with one calix[6]arene and two D2EHPA molecules, based on Job's method. Adenine was recovered from the organic phase by contacting with a fresh acidic solution. (C) 2005 Elsevier B.V. All rights reserved.