It has been well established in the literature that the ratio of the shock wave thickness to the mean free path, (taken at some suitable conditions, e.g. in front of the shock) is independent of the gas density. Our recent experimental results for shocks in perfluoro‐n‐hexane (C6F14) show that this is not always the case.The experiments were performed partly in a large diameter, low‐density shock tube with the use of the electron‐beam densitometer, and partly in a smaller, heated shock tube designed for higher densities. Here the light reflectivity method was used. For higher densities the measured shock thickness, expressed in mean free paths, increases with increasing initial density.The results could be described qualitatively by assuming a preferable orientation of the linearly built C6F14‐molecules. This alignment, which represents a form of rotational relaxation, can be understood in the following way: In the velocity gradient of the shock the molecules are aligned with their longitudinal axis parallel to the direction of the flow. This leads to a broadening of the shock, which depends on its velocity gradient (Mach number) and on the relaxation time of molecule alignment (collision frequency or density).