It is known that the wavefront of a shock wave in a shock tube becomes unstable at particular Mach numbers and unperturbed gas pressure. This instability leads to the distortion of the wave front, i.e. particular Mach numbers define a threshold beyond which the wavefront is no longer plane (see Annou and Ferhat 1997, and references therein). In a recent paper (Annou and Ferhat 1997, subsequently referred to as AF), the authors proposed a microscopic model based on an ionization mechanism described by a set of two reaction–diffusion equations in an effort to interpret the wavefront distortion. Using an analytical approach, they argued that their equations have a bifurcation point beyond which the wavefront instability occurs and that the solution thus obtained describes the distortion of the wavefront. In this brief comment, we present analytical and numerical arguments supporting the fact that the proposed model, although it has an instability point, is unable to explain the front behavior.