A previous analysis of anomalous width variation for a single-ion rarefactive solitary wave [S. S. Ghosh and A. N. Sekar Iyengar, Phys. Plasmas, 4, 3204 (1997)] has been extended to a multi-ion species, and shows that the corresponding width–amplitude characteristics depend crucially on the respective ion concentrations; for example, with increasing light-ion concentration, an ‘increasing- width’ rarefactive solitary wave transforms to a ‘decreasing-width’ one for a constant Mach number exhibiting an overall U-shaped variation profile. The influence of the second ion species generally decreases with increasing amplitude, being maximum for the single to multi-ion transition. However, in the neighbourhood of double-layer-like solutions, the influence again becomes prominent, which may be associated with the corresponding energy–amplitude relations for both the ions.