An experimental investigation on the nucleation density during nucleate pool boiling of saturated pure liquids at low to moderate heat fluxes is described. The surface-liquid interaction during the boiling phenomena and its effect on the nucleation site density and thereby the heat flux is examined. Stainless steel and aluminum with different surface finishes obtained by polishing the surfaces with different grades of emery paper was used in the study. A parameter, Ra, called the arithmetic average roughness or the centerline average was used to characterize the surface micro-roughness. The parameter Ra is defined as the average values of the peaks and valleys on the surface. This was measured experimentally by using a profilometer. The liquids used in the study were distilled water, carbon tetrachloride, n-hexane, and acetone. The nucleation site density at different heat fluxes for various surface-liquid combinations was measured by using high-speed photography. The data showed that the nucleation site density depends on the surface microroughness, the surface tension of the liquid, the thermophysical properties of the heating surface and the liquid, and the wall superheat. A correlation in terms of the wall superheat, the Prandtl number, a surface-liquid interaction parameter (the ratio of the thermal conductivity, density, and specific heat of the solid to the liquid), and a dimensionless surface roughness parameter has been proposed. The correlation proposed matches data obtained in the present study. The correlation also matches data in the literature obtained on copper and nickel surfaces of various surface finishes, further validating the correlation and the mechanism suggested in this study. © Elsevier Science Inc., 1997.