Interfacial momentum transfer was studied in cocurrent and countercurrent wavy-stratified flows in a rectangular channel using both the hot-wire anemometry (HWA) and the non-intrusive photochromic dye activation (PDA) techniques. In cocurrent and countercurrent stratified flows with a wavy interface, the interfacial shear and waves were found to change the kinematic and turbulence structures of the liquid phase significantly. In particular, the mean streamwise velocity profile became nearly constant in the upper half of the liquid phase, and the magnitude of the turbulent streamwise and Vertical fluctuations correlated with the interfacial shear and the wave amplitude, respectively. Numerical predictions of the experimental data were obtained with the k-epsilon model of turbulence. New interface conditions were proposed for wavy flows which yielded predictions in good agreement with the experimental data for both cocurrent and countercurrent flows.