The densities, heat capacities and enthalpies of dilution at 298 K and the osmotic coefficients at 310 K were measured for N,N,N-octyloctyldimethylammonium chloride (OOAC) in aqueous urea solution as a function of urea (m(u)) and surfactant (m(s)) concentrations in order to determine, in comparison with the lower homologue N,N,N-pentyloctyldimethylammonium chloride, OPAC, the influence of the hydrophobic tail on the properties of a double-chain surfactant in the presence of an additive like urea. From specific conductivity data, the critical micelle concentration (cmc) and the degree of the counterion dissociation (beta) have been calculated. From the thermodynamic data, the molar apparent volumes, V-phi, heat capacities, C-phi, and apparent molar relative enthalpies, L phi, were derived as a function of m(u) and m(s) at 298 K.