At nearly 2,900-km depth, the core-mantle boundary (CMB) represents the largest density increase within the Earth going from a rocky mantle into an iron-alloy core. This compositional change sets up steep temperature gradients, which in turn influences mantle flow, structure, and seismic velocities. Here we resolve the thermodynamic parameters of (Mg,Fe)O and compute the melting phase relations of the MgO-FeO binary system at CMB conditions. Based on this phase diagram, we revisit iron infiltration into solid ferropericlase along the CMB by morphological instability and find that the length scale of infiltration is comparable with the high electrical conductivity layer inferred from core nutations. We also compute the (Mg,Fe)O-SiO 2 pseudo-binary system and find that the solidus melting temperatures near the CMB decrease with FeO and SiO 2 content, becoming potentially important for ultralow velocity zones. Therefore, an ultralow velocity zone composed of solid-state bridgmanite and ferropericlase may be relatively enriched in MgO and depleted in SiO 2 and FeO along a hot CMB. ©2019. American Geophysical Union. All Rights Reserved.