The effect of glycyl dipeptides (glycylglycine, glycyl-L-valine, and glycyl-L-leucine) on the micellar properties of gemini surfactant pentamethylene-1,5-bis(dodecyldimethylammonium bromide) (12-5-12) has been investigated by means of conductivity and fluorescence spectroscopy. The results obtained from conductivity show that the effect of glycyl dipeptides depends upon their nature and concentration, as well as the temperature. The values of critical micelle concentration (cmc) of 12-5-12 have been estimated in water + glycyl dipeptide media at various concentrations of dipeptide in the 293.15-318.15 K temperature range at 5 K intervals. From cmc values, it is observed that the micellization tendency of the surfactant increases in the presence of glycyl dipeptides. Thermodynamic parameters (ฮG m o , ฮH m o , and ฮS m o ) of micellization of 12-5-12 in aqueous glycyl dipeptide solutions have been obtained by applying the mass action model and an enthalpyentropy compensation effect was observed. The pyrene fluorescence spectra were used to study the change of micropolarity produced by the interaction of 12-5-12 with glycyl dipeptides, and the aggregation behavior of 12-5-12.