The analytical solution for the mean residence time of a nonreacting gas in a three-zone TAP reactor packed with nonporous pellets is presented. It is shown that the mean residence time is a sum of products of the weighting coefficient and reciprocal of the gas diffusivity of each zone. The weighting coefficients depend on the geometrical characteristics, i.e., lengths and fractional voidages of the three zones. This simple equation is useful for determination of the unknown gas diffusivity in the second zone (catalyst zone) from an experimental diffusion response when diffusivities in the inert-material zones (zone 1 and 3) are known.