The knowledge of the area of interfaces between phases is important to understand and quantify many flow and transport processes in porous media. In this work, we apply the interfacial tracer technique to study the dependence of fluid−fluid interfacial area on saturation and wettability. The interfacial area between the wetting and the nonwetting phases (brine and decane) in unconsolidated porous media (glass beads) was measured using an anionic surfactant (3-phenyl decyl benzene sulfonate) as an interfacial tracer. The beads are water-wet; treating them with organosilane rendered them oil-wet. The measurements were done at a series of steady-state fractional flows, providing data at intermediate as well as residual saturations. Flow rates were kept low so that capillary forces controlled the fluid configurations. We observe significant differences in interfacial areas as a function of wetting-phase saturation as the wettability is changed from water-wet to oil-wet. During primary drainage, measured interfacial area increases monotonically with decreasing water saturation in a water-wet medium. In contrast, the interfacial area measured in the oil-wet porous medium increases with decreasing decane saturation, reaches a maximum, and decreases as the residual decane saturation is achieved. The oil-wet experiment is qualitatively consistent with theoretical results that predict the existence of a maximum in fluid−fluid interfacial area during drainage. The water-wet experiment is consistent with theoretical predictions that include the area of grains in pores that have been drained. We conclude that, in the water-wet experiments, the tracer adsorbs at the interface between the nonwetting phase and the wetting films on grains. In the oil-wet experiments, either the oil films are not sustained at high water saturation or the tracer does not adsorb at them, possibly prevented by steric hindrance. Interpretation of interfacial tracer experiments therefore requires care:  for some mass transport processes, the thin films of wetting phase on grains will not behave the same as macroscopic volumes of wetting phase.