The binding constants of different divalent cations (Ca+2, Mg+2, Fe+2, Hg+2, Ni+2 and Co+2) to oxytetracycline (OTC) has been estimated. OTC can be used in veterinary formulations for the prevention and control of disease and added to feed for such a purpose at a dosage rate of 25-700 mg/kg. Feeds contain Ca+2 and Mg+2 carbonates as raw materials (at a percentage level), and Fe+2 and Co+2 as additives (at a ppm level), but also can contain Hg+2 as a contaminant from fish-based raw materials, or Ni+2 as a residual catalyser used in the partial hydrogenation of seed oils. It is key to develop reliable methods for OTC determination, paying special attention to OTC extraction from feed matrices containing those divalent cations, since the particular structure of OTC contains electron-donor groups able to form strong complexes with such metal ions. The values obtained for K-OTC show that OTC exhibits a large affinity for Ca+2 and Mg+2 and decreases on the increase of the charge/ionic radius ratio for the other divalent cations, what means that Ca+2 and Mg+2 can make more difficult OTC extraction from feeds. (c) 2005 Elsevier Ltd. All rights reserved.