Two-dimensional (2-D) experiments have been conducted to study the phenomenon of liquid entrainment associated with interfacial hydrodynamic instabilities, in particular, the Rayleigh-Taylor instability (RTI). The current work is part of an effort to understand the phenomenon of RTI associated with the rapid expansion of a superheated steam bubble that may occur in a CANDU reactor. The goal of the present work is to quantify the entrainment phenomenon associated with the RTI pertinent to the growth of a 2-D air bubble expanding adiabatically against a 2-D pool of water for a range of operating pressures. This experimental work is similar to that undertaken three decades ago at Massachusetts Institute of Technology, but the geometry has been modified to decrease the blowdown chute volume in order to reduce the experimental uncertainties. The entrainment phenomenon is characterized by means of two parameters that can be used to verify a semiempirical model developed in a parallel modeling effort. Specifically, the first parameter quantifies the width of the mixing zone, and the second parameter quantifies the volumetric ratio between the entrained liquid and the mixing zone. Comparing the experimental data with the model predictions is used to validate the developed model.