We present a theoretical analysis of the interfacial tensions of microemulsions in ternary systems water-oil-nonionic surfactant. For surfactants based on oligo-ethylene oxide head-groups the temperature can be used to tune the properties of the microemulsions. Explicit expressions are derived for the interfacial tension of both droplet and bicontinuous microemulsions. We also discuss the effect of both capillary waves and bending modes on the interfacial tensions. Sottman and Strey [T. Soumann, R. Strey, Ultralow interfacial tensions in water-n-alkane-surfactant systems, J. Chem. Phys. 106 (1997) 8606-8615] have measured interfacial tensions for a range of systems for temperatures covering Winsor I to Winsor III to Winsor II phase behavior. The theoretical expressions are consistent with the experimental data. Under most conditions it is the curvature energy that gives the dominant contribution to the interfacial tension. The magnitude of the effect is determined by different elastic constants for droplet and bicontinuous microemulsions and we conclude that very careful measurements of interfacial tensions can be used to determine both the bending elastic constants of the Helfrich curvature free energy. (c) 2006 Elsevier B.V. All rights reserved. (Less)