The surface roughness on different length scales of In2O3(001) films is detrimental to potential semiconductor applications of this material. One type of surface roughness consists of square shaped islands with edge lengths on the micron scale. This islanding was observed in In2O3(001) films grown on Y-stabilized ZrO2(001), YSZ(001), by plasma-assisted molecular beam epitaxy under oxygen-rich and under In-rich growth conditions. Our reflection high-energy electron diffraction observations during the initial phase of the growth showed that the islanding is caused by a nucleation of the In2O3 with incomplete wetting of the substrate and not by a strain-induced breakup of the film. By systematic variation in the growth parameters Indium-to-oxygen beam flux ratio (VI/III ratio) and substrate temperature we found that the wetting of YSZ(001) by the In2O3(001) film is thermodynamically not preferred and that kinetic limitations, such as low substrate temperatures or (to a lesser extent) high VI/III ratios, were able to force nucleation of continuous films. A growth diagram was constructed that shows the parameter regions with forced or inhibited wetting. The nucleation with inhibited wetting can be applied to the formation of square shaped In2O3(001) islands whose size increases and areal density decreases with decreasing VI/III ratio. To realize continuous, smooth In2O3(001) films under In-rich conditions, a thin nucleation layer was grown at lower substrate temperature to completely wet the substrate. The bu- - lk of the film was grown on top of this nucleation layer at higher temperature to ensure a high crystalline quality.