The interfacial tensions between water- and oil-rich phases in the presence of microemulsions have been measured for ternary systems of water, n-alkanes, and nonionic alkylpolyglycolether surfactants (CiEj). It is found that the minimum of the interfacial tension curve, which is observed for each system in conjunction with the well-known phenomenon of phase inversion, depends sensitively, but systematically, on the chemical nature of the oil and the surfactant. Specifically, the minimum value of the interfacial tension <(sigma)over bar>(ab) decreases by 1 order of magnitude on decreasing either the carbon number of the alkane k by 6, or the number of oxyethylene groups j by 3, or by increasing the number of carbon atoms in the surfactant tail i by 2. The numerical values of the interfacial tensions as a function of temperature an presented along with an empirical description previously suggested [R. Strey, Colloid and Polymer Sci. 272, 1005 (1994)]. From the analysis, in terms of bending energy one obtains estimates for the bending and saddle-splay constants. The similar shape of the interfacial tension curves permits a superposition of the data for all 19 systems in support of a scaling relation recently derived [S. Leitao, A. M. Somoza, M. M. Telo da Gama, T. Sottmann, and R. Strey, J. Chem. Phys. 105, 2875 (1996)]. Furthermore, we note a striking coincidence of the numerical values of critical amplitude ratio R=sigma(0) xi(0)=0.37 kT in near-critical systems and the product <(sigma)over bar>(ab)xi(-2)=0.44(+/-0.10) kT where <(xi)over bar> is the maximum length scale in the bicontinuous microemulsions.