The trace of the force-dipole tensor (A+2B) as well as the components A and B of hydrogen interstitials in niobium were determined from measurements of the lattice parameters of the alpha , alpha ' and beta phases. It is shown that the orthorhombic structure of the beta phase is due to a direct hydrogen-hydrogen interaction. A parameter H, analogous to A or B and representing the strength of this direct interaction, is deduced assuming a central force model. It is shown that the concentration dependence of the structural parameters is correctly predicted by this model. The values of A, B and H are 2.88, 2.75 and 0.05 eV or 2.88, 2.84 and -0.05 eV respectively depending upon whether the nearest-neighbour direct hydrogen-hydrogen interaction is repulsive or attractive.