The study of mass transfer between phases is a very important issue during drilling operations with organic based drilling fluids. The research in the area is necessary to understand the interaction between the formation fluid (oil and/or gas) and the synthetic drilling fluid filtrate inside the invaded zone and also during a kick situation. A comprehensive literature search was conducted in order to identify the state of the art in the subject and to choose the appropriate experimental procedure and physical/mathematical modeling technique. The present experimental work comprises the measurement and modeling of the diffusivity and interface mass transfer coefficients for those gas-liquid systems. The experimental data were obtained with accurate PVT measurements, under high pressure and high temperature conditions. The tests were carried out in the 70°C through 120°C temperature range for n-paraffin liquids with the following mole fraction of gas: i) 15% to 91% mol of CH4 and ii) 27% to 75% mol of CO2. The pressure decay method, which is based on the gas phase pressure decline with time as a consequence of the diffusion process, was applied to data modeling. A transient state diffusion model, with short and long term analytical solution, was applied to determine the mass transfer parameters for the two mixtures: CH4/n-paraffin and CO 2/n-paraffin. 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