Pyrene fluorescence and Krafft temperature measurements have been carried out for various combinations of cationic gemini (m-2-m) with zwitterionic surfactants by changing the length of the hydrophobic tail over the whole mixing range. The results have been evaluated by using the regular solution theory. All the mixtures of cationic gemini + zwitterionic surfactants indicate the presence of synergistic interactions which largely decrease at higher hydrophobicity of both components. A greater amount of gemini component in the mixed micelles induces stronger synergism which reduces with the increase in the length of hydrophobic tail of the gemini component. The Krafft temperature measurements also indicate the presence of strong synergistic interactions, which decrease with increase in the length of hydrophobic tail of both components. (c) 2005 Elsevier Inc. All rights reserved.