When a gas is dissolved into a liquid, the mass transport at the gas-liquid interface depends on many factors including the gas and liquid properties and hydrodynamics. The mass transport of a gas through the liquid is a limiting step in many chemical reactions. To gain a fundamental understanding of multiphase interfaces, the rate of mass transfer has been measured for pure gases (H2, N2, O2, and He) into a thin liquid film with a well-defined surface area and velocity profile. A gravity-driven falling film of liquid with a thickness in the range of 175 to 198 × 10−6 m was contacted with a gas phase inside a closed system. The liquid was circulated continuously and the pressure in the closed system was observed until it was saturated with gas and the equilibrium pressure was reached. In this device, a laminar flow pattern is observed, and therefore, the geometry and hydrodynamics of the liquid film are well defined. This enables the mass transfer rate to be broken down into two parameters; one which is the gas-liquid interfacial area (a), and a second parameter, which is the mass transfer coefficient (kL). Using gases with varying diffusion coefficients enables a comparison of the measured rates of mass transport to the rates predicted by film theory, penetration theory, and Danckwerts’s surface renewal theory.