Intermetallic transition metals silicides are of particular interest because of their specific properties such as high melting point, oxidation resistance, creep resistance and toughness. Single crystals have been obtained for both low and high temperature polymorphs of the compound Nb5−xTaxSi3 (0 ⩽ x ⩽ 5). Although different, their X-ray structures are both solved and refined in the tetragonal I4/mcm space group. The low temperature polymorph belongs to the Cr5B3 structural type (a = 6.5262 Å and c = 11.866 Å for composition Nb3.9Ta16.1Si12) while the high temperature polymorph adopts the W5Si3 structural type (a = 9.9870 Å and c = 5.0794 Å at composition Nb10.3Ta9.7Si12). Structure stability and bonding within the mixed compound Nb5−xTaxSi3 are analyzed comparatively with Nb5Si3 and Ta5Si3 parents, by means of DFT calculations.