In this paper we report the synthesis and structural characterisation of two new K2NiF4-type phases, Ba2In0.5Sb0.5O4 and Sr2In0.5Sb0.5O4. To our knowledge these are the first examples of K2NiF4 compounds of general formula A2MIII0.5M'V0.5O4 with both 3+ and 5+ cations in the octahedral sites. Ba2In0.5Sb0.5O4 is shown to have a tetragonal cell (space group I4/mmm, a=4.1651(1), c=13.299(1) Å) 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) Å). 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.