Knowledge of physicochemical parameters of mixed micelles is important in order to develop drug delivery systems with required characteristics. Investigated bile salts and Tweens are relatively nontoxic surfactants, extensively studied as biocompatible colloidal drug carriers. The micellization behavior of binary anionic-nonionic surfactant mixtures built of sodium cholate or sodium deoxycholate and one of two Tweens (Tween 20 or Tween 60) was investigated by conductivity and surface tension measurements. The results of the study have been analyzed using Clint's, Rubingh's, and Motomura's theories for mixed binary systems. The determined physicochemical properties, particularly the negative values of the interaction parameter, indicate synergism between the individual surfactants in the mixed micelles. It was noticed that Tween with a longer hydrophobic tail shows stronger interactions with selected bile salts. However, it was found that the more hydrophilic bile salt (sodium cholate) generates the stronger synergism with investigated Tweens.