Irradiation of crystalline B10H14 in the beam of a 3-MeV van der Graaf generator, followed by various chromatographic separations, has been developed as a new route to several individual isomers of the conjuncto-borane, (B10H13)2. The high-field (115.5-MHz)11B and 11B-{1H(broad-band noise)} n.m.r. spectra of the previously characterised icosaboranes 2,2′-, 2,6′-, 6,6′-, and 1,5′-(B10H13)2 have been recorded and assigned. Comparison with the spectra of three new isomers isolated from the u.v. photolysis of B10H14 in cyclohexane solution and from the high-energy electron bombardment of solid B10H14 has led to further structural characterisations: two of the new isomers are identified as 1,2′- and 2,5′-(B10H13)2 and the third is 5,5′-(B10H13)2 or its meso-diastereoisomer 5,7′-(B10H13)2. Analytical high-pressure liquid chromatography has been used to compare the isomeric composition of mixtures of icosaboranes produced by the above routes and by the catalytic thermolysis of B10H14, and the results permit further discussion on possible mechanisms of formation of bi(nido-decaboranyl). The melting points of individual isomers have been determined as follows: 1,2′ 139–142; 1,5′ 114–115; 2,2′ 178–179; 2,5′ 109–111; 2,6′ 154–155; 5,5′(or 5,7′) 97–98; and 6,6′-(B10H13)2 198–199 °C.