A simple model based on (a) the formation of a deposit under critical conditions and (b) on a boundary layer growing along a hollow fiber or tubular membrane in cross-flow filtration suggests that the limiting and critical fluxes can be theoretically linked. With certain assumptions, it is shown that the critical flux is equal to 2/3 of the limiting flux. The assumptions made and the existence of a direct relationshiop between critical and limiting flux in "real world" membrane process are further discussed. (C) 2003 Elsevier B.V. All rights reserved.