The ion pair formation constants at infinite dilution (K(IP)(0)) of bis(trifluoromethanesulfonyl)amide ([NTf(2)]-()) salts in dichloromethane were determined conductometrically at 298.2 K. The cations used were nine which was explained on the basis of the structures of the cations For the salts used, the fundamental properties as ionic liquids, such as density, viscosity, and electric conductivity, were also measured The activation energies for viscous flow were determined from the temperature dependence of the viscosity, and their relation with the K(IP)(0) values was discussed