A very simple derivation is given of the ionization probability P − for negative ions sputtered off a metal surface. The result P −≈e−(ϕ−A)/cv⊥ shows the experimentally observed dependences on work function ϕ, atomic affinity A, and outwards velocity v ⊥. Estimates of c show good agreement with experiment, too. The physical picture behind the process is discussed. © 1983 IOP Publishing Ltd.