Purely viscous and purely elastic interfacial instabilities as well as the layer encapsulation phenomena in superposed pressure-driven channel flow of well characterized Newtonian and viscoelastic fluids have been studied. We have characterized both the base and the perturbed flows by measuring velocity profiles and encapsulation rates as well as determining the stability of the interface. Specifically, it has been shown that above a critical depth and viscosity ratio, encapsulation occurs and the rate of encapsulation is a strong function of viscosity ratio at a given depth ratio. In addition, for the first time purely viscous and purely elastic interfacial instabilities in superpose plane Poiseuille flows have been experimentally observed. Specifically, it has been shown that the incipient instability is two-dimensional and well described by theoretical predictions using linear stability theory with a constitutive equation that can accurately describe the rheology of the test fluids. Moreover, it has been demonstrated that in absence of encapsulation and significant second normal stresses in the test fluids the interfacial waves are nearly two-dimensional in the weakly non-linear regime. (C) 2000 Elsevier Science B.V. All rights reserved.