The adsorption and equilibrium surface phases of Eu on graphene on Ir(111) were investigated in the temperature range from 35 to 400K and for coverages ranging from a small fraction of a saturated monolayer to the second layer by scanning tunnelling microscopy. Using density functional theory including 4f-shell Coulomb interactions and modelling of electronic interactions, excellent agreement with the experimental results for the equilibrium adsorbate phase, adsorbate diffusion and work function was obtained. Most remarkably, at 300K in an intermediate coverage range a phase of uniformly distributed Eu clusters (10 to 20 atoms) coexisted in two-dimensional equilibrium with large Eu-islands in a (√3 x √3)R30° structure. It was argued that the formation of the cluster phase was driven by the interplay of three effects. Firstly, the metallic Eu-Eu binding led to the local stability of (√3 x √3)R30° structures. Secondly, electrons lowered their kinetic energy by leaving the Eu clusters, thereby doping graphene. Thirdly, the Coulomb energy penalty associated with the charge transfer from Eu to graphene was strongly reduced for smaller clusters.Phase Coexistence of Clusters and Islands: Europium on Graphene. D.F.Förster, T.O.Wehling, S.Schumacher, A.Rosch, T.Michely: New Journal of Physics, 2012, 14[2], 023022