Commercial manganin gauges were used in a series of planar impact experiments in order to determine the alpha to or from epsilon phase transformation in Armco iron. Using calibration curves for shock levels, the direct alpha to epsilon transformation stress for 5mm thick samples was found to be 136.5+or-2.5 kb when impact loaded to stresses between 145-180 kb. With the aid of a special calibration curve for the release part of the stress pulse the onset of the reverse epsilon to alpha transformation was found to be at a stress of 100+or-2.5 kb. The specific volume of the iron at this transformation point was calculated, with the aid of the measured lagrangian velocity of the rarefaction shock, to be at about 0.9 the initial volume.