Abstract A simple model is proposed to test the applicability of Sigmund's theory to polycrystalline copper targets. A fixed amount of fortuitous chan = nelling is allowed in the target and the sputtering coefficient is calculated by using a combination of single crystal sputtering theory and Sigmund's for= mula. The results indicate that the effect of channelling is to shift the sputtering maximum to higher energy values than predicted by the amorphous target theory. Furthermore, the form of the sputtering yield-energy curves deviate significantly from the Sigmund sputtering curve.