Axial flow cyclones are particularly attractive in both gas cleaning and aerosol sampling due to the axial alignment between the inlet and outlet. In order to aid the development of suitable instruments for aerosol sampling, a mathematical model of axial flow cyclone performance under laminar flow conditions has been developed, using a number of simplifying assumptions. Aerosol penetration is considered separately through the vane section of the cyclone and: its body, and the two models combined. Relating all dimensions to vane Pitch, penetration as a function of Stokes＇ number in the cyclone vanes is predicted to be dependent on vane outer radius, vane inner radius and the number of vane turns. Penetration in the cyclone body is predicted to depend on body length alone. Encouraging agreement is seen between published experimental data for aerosol sampling cyclones and predicted (50)d(ac) values. Crown