The efficiency with which particles are collected by impaction from a round jet of diameter, W, onto a collecting plate at a distance S from the jet orifice has been measured as a function of the particle Stokes' number for values of S/W between 0·5 and 10·0. The Stokes' number corresponding to a collection efficiency of 0·5 varied between 0·17 and 0·24 for values of S/W up to 8·0.