This paper focuses on the development of natural convection mass transfer induced by gas diffusion in a liquid-saturated vertical porous column. The porous column is confined with two impermeable end faces and an isosolutal side surface. Mass transfer by natural convection is studied by numerically solving the conservation equations of mass, momentum, and diffusing species. To describe the transient behavior of natural convection, transient and integrated Sherwood numbers, i.e., Sh(t) and Sh(i), are defined on the basis of a parallel pure diffusion case. The evolution of the natural convection process is systematically studied for the aspect ratios ranging from 0.125 to 8 and Rayleigh numbers from 10 to 1000. It is found that the Sherwood number increases with increasing Rayleigh number, whereas it increases first and then decreases with increasing aspect ratio. The results confirm the assumption in a previous study that the effect of natural convection is negligible on the diffusion process in the gas effective diffusion coefficient measurement,(1) and also provide important implications on how to prepare core samples in laboratory for gas effective diffusion coefficient measurements.