The development of biological targeting agents such as proteins, peptides, antibodies and nanoparticles with a range of biological half-lives demands the production of new radionuclides with half-lives (physical) complementary to these biological properties. Zirconium-89 (89Zr) is a promising radionuclide for development of new immuno-PET agents due to its convenient half-life of 78.4 h, β+ emission rate of 23%, low maximum energy of 0.9 MeV resulting in good spatial resolution, stable daughter isotope of Yttrium-89 (89Y) and favorable imaging characteristics, with only one significant γ-line of 909 keV emitted during decay alongside the 511 keV positron photons. Our aim was to prove that isotopically pure 89Zr could be produced in an IBA Cyclone 18/9 cyclotron equipped with a COSTIS STS using the 89Y(p,n)89Zr reaction and optimise the yield by reducing the beam degrader thickness without producing either 88Zr or 88Y. The degradation of the beam energy with 400 and 500 μm thick Niobium foils were achieved...