The aggregation behavior of pure cationic surfactants (quaternary salts) in water has been studied by electrical conductivity (at 293.15-333.15 K), surface tension, dye solubilization and viscosity measurements (at 303.15 K). Critical micelle concentrations (CMCs), degree of counter ion dissociation (P), aggregation number and sphere-to-rod transition for cationic surfactants are reported. Using law of mass action model, the thermodynamic parameters, viz. Gibbs energy (Delta G(m)degrees), enthalpy (Delta H(m)degrees) and entropy (Delta S(m)degrees) were evaluated. The plots of differential conductivity, (dk/dc)(T,P), versus the total surfactant concentration enables us to determine the CMC values more precisely than the conventional method. Surfactants with longer hydrocarbon chain are adapted to rodlike micelle better than to a spherical micelle. The data are explained in terms of molecular characteristics of surfactants viz. nonpolar chain length, polar head group size and counter ion. (c) 2004 Published by Elsevier B.V.