We present ab initio calculations of the zerotemperature composition dependent spin transition pressures in rocksalt (B1) (Mg1x,Fex)O. We predict that the spin transition pressure decreases with increasing Mg content, consistent with experimental results. At highpressure, we find that the effective size of Mg is smaller than high-spin Fe but quite close to low-spin Fe, consistent with a simple compression argument for how Mg reduces the spin transition pressure. We also show that the spin transition is primarily driven by the volume difference between the high-spin and low-spin phases, rather than changes in the electronic structure with pressure. The volume contraction at the transition is found to depend non-monotonically on Fe content. For FeO we predict a B1 ! iB8 transition at 63 GPa, consistent with previous results. However, we also predict an unexpected reverse transition of high-spin iB8 ! lowspin B1 at approximately 400 GPa. Citation: Persson, K., G. Ceder, A. Bengtson, and D. Morgan (2006), Ab initio study of the composition dependence of the pressure-induced spin transition in the (Mg1x,Fex)O system,