Oxide ceramic matrix composites (OCMCs) are promising candidates for high- temperature applications, e.g. in the aerospace industry or for furnace and turbine parts, as they combine a damage tolerant fracture behavior with the advantages of monolithic ceramics. These include a low density in comparison to metallic components and a high corrosion resistance. OCMCs are dominated by the properties of the ceramic fibers, their mechanical properties and high-temperature behavior are therefore of crucial importance. At the DITF Denkendorf oxide ceramic fibers based on alumina (α-Al2O3) and mullite (Al2O3·SiO2) have been developed on a pilot scale. While alumina fibers are characterized by their high tensile strength and Young’s modulus, mullite fibers show a lower tensile strength but an increased high-temperature stability and creep resistance. The stability of both fiber types is limited by grain growth during long-term exposure to high temperatures, which leads to embrittlement and thus a reduction in the fiber strength. In this work, oxide ceramic fibers with novel phase compositions were developed to reduce grain growth and increase the high-temperature stability of alumina- and mullite-based fibers. Zirconia (ZrO2) is known to limit grain growth in bulk ceramics and to enhance the fracture toughness, e.g. by transformation toughening. We studied the influence of different amounts of ZrO2 on the phase formation processes in alumina and mullite fibers and characterized the properties of the resulting zirconia-toughened alumina (ZTA) and mullite (ZTM) fibers using high-temperature DSC/TGA, X-ray diffraction (XRD), density measurements, scanning electron microscopy (SEM) and mechanical tests on single filaments. Ceramic fibers were developed in a four-step process. First, water-based spinning dopes containing ceramic precursors and a polymer were synthesized and characterized using rheology. Suitable spinning dopes were then selected for a dry-spinning process to gain green fibers with 90 and 500 filaments. Green fibers were calcined to remove volatile components before the final sintering step. Calcination and sintering were carried out continuously in air atmosphere at temperatures between 800-950 °C and 1400-1500 °C, respectively