The nucleation site density (N/A) during pool boiling of binary mixtures has been experimentally examined in terms of the heating surface physical properties, the liquid physical properties, the vapour liquid equilibria and the surface micro-roughness. The liquid mixtures used were acetone - carbon tetrachloride and n-hexane - carbon tetrachloride. A parameter Ra, to characterize the micro-roughness of the heating surface was defined as the average value of the peaks and valleys on the surface. The study shows that while at low Ra values the physical properties of the liquid and the binary composition are not as important as the surface roughness; at higher Ra values the liquid properties and composition are also important. A correlation based on the present data has been proposed.