The secondary-ion intensity of sputtered Si has been measured as a function of the emission energy using a previously calibrated mass analyser. From the Sigmund - Thompson energy distribution for neutrals, the ionization probability for ions is inferred. It is found that the behaviour of at high emission energies is consistent with neutralization via the electron tunnelling mechanism (resonant electrons tunnelling from the substrate to the outgoing ions). The possibility of electronic excitations induced by the collision cascade in Sroubek's model is also considered.