As part of a study of low work function coatings for enhanced electron thermionic and field emission, very thin films of ThO2 (similar to 40 nm thick) grown by physical vapor deposition on polycrystalline Ir have been characterized by Auger electron spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy. Following resistive heating to >1750 K strong morphological heterogeneity was observed in the ThO2 film suggesting significant sensitivity to local surface variations in the polycrystalline Ir substrate. Observed growth paradigms include: step-flow, quasi-laminar and triangular hillocks; pyramidal structures on large terrace regions; highly anisotropic growth and coalescence of pyramids; and overlapping regions where multiple growth modes exist. Despite the variety of morphological structures, XRD confirmed a preferred (111) orientation for the ThO2 fluorite crystallites, which is in good agreement with theoretical predictions and previous work on ThO2 films. Further, analogous behavior of calcium fluorite thin films, where similar morphological features were observed, has been reported. Among the morphologies in the ThO2/Ir system, quasi-laminar growth suggests the possibility of improving the efficacy of very thin films of ThO2 for enhanced electron thermionic and field emission. Published by Elsevier B.V.