In this paper the synthesis and structural characterisation of two new K2NiF4-type phases, Ba2In0.5Sb0.5O4 and Sr2In0.5Sb0.5O4, are reported. These are the first examples of K2NiF4 compounds of general formula A(2)M(0.5)(III)M(0.5)('V)O(4) with both 3+ and 5+ cations in the octahedral sites. Ba2In0.5Sb0.5O4 is shown to have a tetragonal cell [ space group I 4/mmm, a = 4.1651( 1), c = 13.299( 1) Angstrom] with an essentially disordered arrangement of In and Sb. In the case of Sr2In0.5Sb0.5O4, however, ordering of In and Sb is observed leading to an expanded unit cell [Pmcb, a = 5.7592(1), b = 5.7740( 1), c = 12.543(1) Angstrom]. The results therefore show that varying the size of the alkaline earth cation has a pronounced effect on the ordering of In and Sb within the structure.