Modular multilevel converters are gaining attention since medium voltage direct current grids have been proposed as the new key technology for the future transmission and distribution systems. The design of such systems need to address several challenges and this paper presents a thermal study of a modular multilevel converter full-bridge IGBT based submodule. Thorough tests have been performed under severe ambient thermal conditions and electrical stresses, typical to regular modular multilevel converter operation. Presented results demonstrate an excellent match between theoretically predicted numbers and those obtained in a custom-designed experimental test setup.