The gas-phase pyrolysis of bis(9-borabicyclo[3.3.1]nonane) (I) has been studied in the range of 400-620 Deg. At 490 Deg approx. equal amts. of 1-borabicyclo[4.3.0]nonane (II) and 8-borabicyclo[4.3.0]nonane (III) are formed (yield >95%). Heating III, which presumably exists as the dimer with a 1,1:2,2-diborane(6) structure or as an oligomer with mixed 1,1:2,2- and 1,2:1,2-diborane(6) structure, in the condensed phase to above 100 Deg results in the formation of the air and hydrolysis stable IV with a 1,2:1,2-diborane(6) structure. The structure of IV was detd. by X-ray anal. The pyrolysis of 9-alkyl-9-borabicyclo[3.3.1]nonanes (V, alkyl = Et, 1,2-dimethylpropyl, isopinocampheyl) gives II and small amts. of I. The formation of II from I as well as II and I from V indicate that the monomer of I is an intermediate.