Many elements are rapidly released from oxide matrices. Some oxide powder targets show a fast sintering, thus losing their favorable release characteristics. Loosely packed oxide fiber targets are less critical since they may maintain their open structure even when starting to fuse together at some contact points. The experience with various oxide fiber targets (titania, zirconia, ceria and thoria) used in the last years at ISOLDE is reviewed. For short-lived isotopes of Cu, Ga and Xe the zirconia and ceria targets respectively provided significantly higher yields than any other target (metal foils, oxide powders, etc.) tested before. Titania fibers, which were not commercially available, were produced in a relic process by impregnation of a rayon felt in a titanium chloride solution and subsequent calcination by heating the dried felt in air. Thoria fibers were obtained either by the same process or by burning commercial gas lantern mantle cloth. In the future a beryllia fiber target could be used to produce very intense 6 He beams (order of 10 13 ions per s) via the 9 Be(n,α) reaction using spallation neutrons.(Submitted to Nuclear Instruments and Methods in Physics Research B) Introduction A great variety of materials is used as ISOL (isotope separation on-line) targets (see e.g. [1]): molten metals, solid metals (as foils or powder), carbides, oxides and other compounds. Here we will discuss recent on-line results (release parameters and yields) of ISOLDE oxide targets and, in particular, of oxide fiber targets.Oxide targets have a long history at ISOL facilities. Already in the very first ISOL experiment [2] a 10 kg UO 2 target was used. Then, in the first experiments at ISOLDE several oxide targets served to produce radioactive noble gas isotopes [3,4]. These oxides (TiO 2 , ZrO 2 , CeO 2 and ThO 2 ) were precipitated out of solutions, vacuum-dried at room temperature and ground to powder. The targets were only partly dehydrated when operated at room temperature and had average delay times (for Kr and Xe) of 17 − 110 s [5].Later, other oxides were used at higher temperatures, after having been dehydrated by slowly heating them off-line under vacuum. All these oxides were used in form of powder or as pills of pressed powder: BeO, MgO, Al 2 O 3 , CaO, SrO, BaO, La 2 O 3 , ThO 2 and UO 2 . However, a powder target has a large number of mutual contact points between the individual grains. When heated for longer time to high temperature, the grains can "bake together" at these contact points and the formerly fine-grained powder will sinter to a bigger block with consequently longer diffusion paths and slower release. This was observed in particular for thoria and urania powder targets. Here it would be preferable to use instead thin fibers, which are arranged as a loose "pile of spaghetti". They have less mutual contact points and provide a better mechanical stability even when sintering starts locally at the contact points.Felts and cloths made from fibers of ZrO 2 and other oxides are commercially av...