The compounds LnSrScO4, where Ln=La, Ce, Pr, Nd and Sm, have been synthesized. Rietveld profile analysis of powder X-ray diffraction data collected at room temperature reveal that the compounds possess a modified K2NiF4-type structure with orthorhombic cell symmetry formed by tilting of the ScO6 octahedra. Variable temperature (25–1200 °C) powder X-ray diffraction data show that at the highest temperatures the structures of LaSrScO4 and PrSrScO4 transform to the regular tetragonal K2NiF4-structure type but the degree of orthorhombicity (c/a) in the unit cells initially increases on heating for all materials, reaching a maximum near 300 °C. This structural behavior is analyzed in terms of relative ionic radii of the various lanthanides and scandium. A general structural model based on tolerance factors has been developed for the family of materials A2BO4 with various A and B cation sizes.