An experimental investigation is presented for the boiling of distilled degassed water at atmospheric pressure on a single copper surface with different surface finishes, in which the activation of nucleation cavities under the influence of different levels of heat flux was investigated. The measured active cavity radii showed good agreement with the active cavity radii predicted by the Hsu model. As a result of this investigation, it would appear that smaller cavities are better able to entrap vapor residue and be activated by their neighbors when boiling is initiated on the surface than larger cavities and that the reason for the relatively high levels of superheat at incipience commonly encountered is the nonexistence of sufficiently large cavities with the prerequisite residue of vapor. The correlation suggested by Brown relating the active site density (N/A)rc as a function of cavity radius rc in the form (N/A)rc α (1/rc)m was confirmed. On the basis of this work Griffith and Wallis’ recommendation that the parameter group [2σ Ts/ρv hfg (Tw − Ts)] be used to characterize a boiling surface was substantiated.