Hitherto, most discussions of nucleation phenomena have been concerned with the condensation of vapors. In this paper, thermodynamic arguments are presented to show that such classical nucleation models are inadequate for interpreting nucleation in aqueous media The use of the Young equation involving force balance solutions is applicable only for uncharged interfacial systems. To avoid the involvement of a contact angle concept, which provides little kinetic information about heterogeneous nucleation, the present paper is concerned with the use of interfacial measurements to calculate the rates of these reactions. The method is illustrated by calculation of a rate of heterogeneous nucleation of hydroxyapatite at elastin surfaces (4.4 x 10(-8) mol min(-1) m(-2)) that is close to the value, 3.9 x 10(-8) mol min(-1) m(-2), reported in the literature.