The crystal structures of eight tetragonal, gillespite-structured phases in the effenbergerite(BaCuSi4O10)–wesselsite (SrCuSi4O10) solid solution (Sr1−xBaxCuSi4O10, where x is the mol fraction of the Ba end-member), have been refined from powder, neutron time-of-flight, diffraction data. The accommodation of the larger, more electropositive Ba2+ cation within the crystal structure of SrCuSi4O10 is achieved by increasing the magnitude of the rotation of the square-planar CuO4 group about the c axis, coupled with an anti-phase rotation, and concomitant tilting, of the Si4O10 polyhedral unit. To an excellent approximation, these structural changes are equivalent to a rigid sphere substitution, the radius of which is given by the compositionally averaged ionic radii of Sr2+ and Ba2+. The compositional-dependence of the lattice parameter c is significantly larger than that for a at low values of x, and is particularly well parameterised in terms of the variations of the calculated ionic radius of the alkaline-earth site and the observed tilt of the SiO4 tetrahedron. The lattice parameter a exhibits a negative deviation from Vegard’s rule resulting from the more complex, coupled structural response to the change in the effective ionic radius at the Sr/Ba site.