AbstractAlthough considerable work has been done on the problem of heat transfer radially in fixed beds through which gases are flowing, the data available for mass transfer are limited to one pipe size and one packing size and refer to average diffusivities for the entire bed. The present study was undertaken to determine: (1) diffusivities over a range of pipe and packing sizes and (2) the effect of radial position in the bed.The measurements were made by introducing carbon dioxide into an air stream and analyzing the resultant mixture at various positions in the bed downstream from the point of injection. Pipe sizes of 2, 3, and 4 in. were packed with spherical particles of 5/32‐, 1/4‐, 3/8‐, and 1/2‐in. nominal diameter.The differential equation describing the concentration in a packed bed when diffusivityEand the velocityuare permitted to vary with radial position was solved by use of an I.B.M. card‐programmed calculator for the computations.The results show that the Peclet numberDpu/Eincreases from the center towards the wall of the pipe and that the increase is significant whenDp/Dtis greater than 0.05. Empirical correlations are then presented for both point Peclet numbers, which vary with radial position, and average Peclet numbers for the entire bed.The variations in Peclet number with radius can be explained in terms of the corresponding variation in void fraction for 81% of the radius of the bed. At modified Reynolds numbers above 40 to 100 the equationPe= 8.0 + 100 (δ — δ0) correlates the effects of pipe and packing size and radial position. At radial positions greater than 0.81 wall friction influences turbulence conditions and the