Specific conductivities, densities, heat capacities, and enthalpies of dilution at 25‡C were measured forN,N,N-octylpentyldimethylammonium chloride (OPAC) in water-urea mixtures at various urea concentrations mu as functions of the surfactant concentration ms. From conductivity data, the cmc and the degree of the counterion dissociation Β of the OPAC micelles were calculated. The cmc increases linearly with increasingmu while Βvs. mu is a smooth concave curve. From the experimental thermodynamic data, the apparentYΦ and partialY2 molar properties (volumes, heat capacities, and relative enthalpies) are derived as functions of mu andms. The effect of urea on the dependences of the different properties on ms are discussed. From data in the premicellar region the standard partial molar volumesV20 and heat capacitiesCp20 were evaluated. It was observed thatV20 increases linearly withmu whileCp20 decreases. The properties of OPAC in the dispersed and micellized forms at the cmc were obtained and, therefore, the thermodynamic functions of micellization were calculated on the basis of the pseudo-phase transition model.