The α − E polymorphic phase transition is known to occur in some steels. In this work 080M40 plain carbon steel bright round rolled bar, was machined and heat-treated to produce an annealed or a hard martensitic structure. These samples were shock loaded, by plate impact, and the material response recorded using in-situ manganin stress gauges and heterodyne velocimetry. The Hugoniot elastic limit for the annealed samples was determined to be (2:1 0:3)GPa. The Hugoniot elastic limit was not well de ned for the hardened samples. The phase transition pressure was determined to be (13:6 0:3)GPa and above 14:8GPa for the annealed and hardened samples respectively.