Reactions of Cookson's diketone with potassium halides in the polyphosphoric acid medium Aim. To study the rearrangement of Cookson's diketone by the action of potassium halides under conditions of polyphosphoric acid catalysis. Results and discussion. Chemical behaviour of Cookson's diketone (C S-trishomocubane-8,11-dione) in the reactions with potassium halides (KCl, KBr, KI) in the polyphosphoric acid (PPA) medium have been studied. When treated with the KI/PPA mixture Cookson's diketone undergoes reduction leading to tetracyclo[6.3.0.0. 4,11 .0 5,9 ]undecane-2,7dione. The use of KBr instead of KI leads to formal addition of HBr to the cyclobutane ring of C S-trishomocubane-8,11-dione and gives 3-bromotetracyclo[6.3.0.0 4,11 .0 5,9 ]undecane-2,7-dione. The general scheme of the cycle opening mechanism has been proposed. In the case of using the KCl/PPA mixture the reaction does not occur. Experimental part. The structure and composition of compounds were proved by the methods of 1 H and 13 C NMR-spectroscopy, and also X-ray diffraction analysis. Elemental analysis was performed for the compounds obtained. Conclusions. It has been shown that hydrohalic acids generated in situ under the reaction conditions do not induce the rearrangement of Cookson's diketone to the D 3-trishomocubane system. The cyclobutane ring opening and reduction take place instead.