The problem of steady-state diffusion of aerosols without axial diffusion inside a long annular channel and for a small diffusion parameter, mu,is treated in an analytic form. A solution for laminar how without a velocity boundary layer is given and the results are compared with existing numerical approaches. Comparisons of the collection behaviour between a hat rectangular channel a circular tube and an annular channel are made through dimensionless efficiency curves for different ratios R(1)/R(2).