Abstract : Bands of intense shear have been reported in steels which have been subjected to a high strain rate. It was proposed that these bands were caused by unstable adiabatic shear; that the rate of thermal softening (as heat is generated locally by plastic work) is greater than the rate of work hardening of the metal. Thus shear continues preferentially in these soft bands and, if the deformation leads to fracture, then fracture usually occurs within these bands. On metallographic examination the bands appear white when etched in picral or nital, and have always been assumed to consist of martensite. Work was undertaken to check this assumption. (Author)