Nucleation, growth and thermal stability of Pt particles supported on well orderedFe3O4(111) thin films grown on Pt(111) were studied by scanning tunnelling microscopy (STM) andtemperature programmed desorption (TPD) of CO. STM studies showed that Pt growsthrough the formation of single-layer islands that coalesce at high coverage. Vacuumannealing at 600 K caused Pt sintering and the formation of extended two-dimensional (2D)islands one and two layers in thickness at sub-monolayer coverage. Well faceted,three-dimensional (3D) Pt nanoparticles formed by annealing to temperaturesabove 800 K were encapsulated by an FeO(111) monolayer. These results wererationalized in terms of the high adhesion energy for Pt on iron oxide surfaces. COTPD studies showed that 2D structures, formed at 600 K, exhibit much lower COadsorption capacity as compared to the Pt(111) single crystal surface. This effecthas been tentatively assigned to lattice expansion in the Pt 2D islands leading toweakening of the Pt–CO bond due to reduction of the back-donation.