Measurement of gas diffusivity in reservoir fluids is of great interest for a number of applications, and among different methods for the measurement, the Pressure Decay method has received special attention due to its simplicity. In this technique, a non-volatile quiescent oil column is brought in contact with a diffusing single component gas from the top and the rate of change of gas pressure in the gas cap is recorded. The interpretation of outcomes is based on solution of a forward problem, which sometimes invokes a complicated boundary condition. In this work, an analytical solution is presented for the most general form of the boundary condition which models the interface. It takes into account all mass transfer key parameters including gas solubility, a diffusion coefficient and a possible interfacial resistance. The effect of resistance against mass diffusion at the interface is usually neglected in modeling. Through this solution, the role of interface resistance is better explained and one can realize how the resistance exactly affects the diffusion process. A detailed sensitivity analysis of each parameter is conducted and specifically in the case of interface resistance, it is illustrated that a numerical value can be reported for the interfacial resistance while it does not affect or hinder the diffusion process physically. This could unnecessarily increase the degree of freedom of the backward problem, and may lead to misleading parameter estimation results (despite a good match of the measurements). Using our new interface boundary condition reveals that some of the previous works on the modeling of interface resistance are subject to underestimation of the rate of gas dissolution which may lead to erroneous estimation of parameters. (C) 2012 Elsevier Ltd. All rights reserved.