Polycrystalline Znl(1-x)Co(x)O magnetic semiconductors were synthesized by hydrothermal technique from aqueous solutions of Zn and Co acetates using potassium hydroxide KOH as hydrolytic catalyst. The structural and magnetic properties of the samples were investigated as a function of acetates and KOH concentrations. The results show that the highest amount of Co that can be successfully inserted in the ZnO matrix is around 10%, when using concentrations of 0.5 and 0.77 mol/L of acetate and KOH, respectively. For the Zn0.9Co0.1O sample, X-ray diffraction indicates a pure ZnO wurtzite structure free of parasitic phases in agreement with optical measurements that reveal the presence of CO2+ cations in substitution of Zn2+. The magnetization measurements show a paramagnetic behaviour and no evidence of ferromagnetism. This may be due to the absence of free carriers. (c) 2005 Elsevier B.V. All rights reserved.