Defect formation upon reactive direct-current magnetron sputtering of GeO2 films has been studied using x-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive x-ray (EDX) analysis, as well as atomic force microscopy and optical microscopy. The density of the defects shows a clear correlation with oxygen flow rate. High defect densities are observed for flow rates above approximately 27.5 seem, where a transition of film structure is observed. XRD analysis identifies the defects as Ge crystallites. Defect formation is intimately related to the formation of whiskerlike structures on the target in the vicinity of the erosion zone and arcing in the same area. SEM imaging in conjunction with EDX allows a detailed understanding of defect formation.