In the combustion of pulverized char particles, the boundary layer diffusional resistance has an important effect on the overall reaction rate. However, the equations, which are commonly used in practice, are those for special cases obtained after making simplifying assumptions about the governing differential equation, and thus involve certain inaccuracies. Therefore, we derive from the Maxwell-Stefan equations an analytical expression for the mass transfer coefficient for oxygen reacting with a small carbon particle in an atmosphere of O-2, N-2, CO2, CO, and H2O. The result is given as a correction factor to the baseline case of oxygen diffusing in a stagnant gas film with all binary diffusion coefficients being equal and neglecting Stefan flow. It is shown that the correction can be up to 17%, depending on the primary reaction product (CO or CO2), reaction rate, and gas-phase composition. The approximate, expression obtained for the mass transfer coefficient has a simple form and can be used in engineering calculations.