New Hugoniot data on polycrystalline (avg.porosity 6.9%) samples of the magnesiowustite Mg_(0.6)Fe_(0.4)O are presented, covering the pressure range up to 200 GPa. When our data are fit by a single 3rd order Eulerian Hugoniot with K_0 constrained to its ultrasonic value of 161.5 GPa, the required isentropic pressure derivative K_0′ is 4.37 +/− 0.37. This value is significantly lower than the ultrasonic one of 6.18; existing isothermal compression data, however, are in agreement with our value rather than the ultrasonic one. Our data are adequately explained without phase transitions. There is some marginal evidence for a possible phase transition around 120 GPa. If such a transition indeed occurs it is probably of small volume change compared to the transition observed in FeO; we place an extreme upper bound of 3% on the density change such a transformation could involve and still be consistent with the data. Contrary to earlier hypotheses, we believe that a phase transition in magnesiowustite is not a likely explanation of the seismic effects in the D″ region of the lower mantle. The wustite transition may be a more complex phenomenon than initially supposed — perhaps an effect of nonstoichiometry localized to the iron-rich end of the solid solution series
