Gas-to-particle mass transfer is studied analytically as well as numerically. Porous particles are modeled as a homogeneous assembly of sorbent material, forming a spherical, macroporous structure. The concentration in the macropores and in the solid is obtained as a function of time and space. The “Langmuir” theory is used to model sorption kinetics. Results show that, over a wide time range, the concentration in the solid is negligible, and the macropore concentration reaches a pseudo-steady state. For that case an analytical expression is derived for the macropore concentration inside the particle and at the particle surface in particular. It is shown that the surface concentration decreases with decreasing Biot numbers and increasing Thiele numbers. The analytical model discussed in this work can be utilized in computational mass transfer studies in lieu of the “perfect sink” assumption, in which the surface concentration is identically zero. Moreover, it captures the effects of enhanced mass transfer due to convection at the gas–particle interface.