The model for countercurrent steady state mass transfer between uniform dispersed phase particles and a continuous phase, both assumed to move in plug flow, with diffusion resistance inside the particles, and film resistance around the particles, is solved anal. for three particle geometries: infinite plates, infinite cylinders and spheres. In the soln., the direct relation between the local continuous phase concn., and the av. dispersed phase concn. is accounted for in the boundary condition. Concn. profiles in the mass exchanger are derived from the anal. soln., as well as asymptotic Sherwood nos. The anal. soln. requires the detn. of roots of the characteristic equations, which is cumbersome for spherical particles. Correlations are provided for the asymptotic Sherwood nos. for easy computation. The model can easily be used for the analogous direct heat exchange problem.