The synthesis and thermal bromination of octahydropentalene was studied. The reaction afforded 1a,3a,4b,6b-tetrabromo-1,2,3, 4,5,6-hexahydropentalene (14) with remarkable regio- and stereospecificity. The structure of the product was determined by H-1 and C-13 NMR data and single X-ray structural analysis. The treatment of octahydropentalene with tenfold bromine gave the octabromopentalene derivative. The formation mechanism of the products is discussed. (c) 2005 Elsevier Ltd. All rights reserved.