Abstract When low-energy ions are incident on a solid surface, a large fraction is neutralized via an Auger or resonance process. Some useful information on details of the neutralization process can be obtained by studying the ion fraction of backscattered particles. An accurate evaluation of the characteristic velocity, which determines how fast the ion fraction varies with the incident ion velocity, is expected to be particularly useful. In this paper we give a theoretical analysis of experimental ion yield data that exist in the literature. It is shown that a successful interpretation of the ion yield data can be made by taking into account the variation of the characteristic velocity with respect to the incident ion energy.