Sr1−xCaxCrO3−δ (x = 0.25, 0.50, 0.75) solid solutions have been synthesised using the ‘hard–soft’ technique and structurally characterised. Sr1−xCaxCrO3 phases were prepared under ‘hard’ high pressure and high temperature synthesis and then reduced at ‘soft’ low temperature conditions. Analysis by powder X-ray diffraction and Rietveld refinements reveals that the reduction of each composition gives two crystalline products, with 6H and 15R structure types previously reported in SrCrO3−δ. analogues. These results demonstrate that the 6H and 15R structures are stable for very high calcium contents up to x = 0.75 in Sr1−xCaxCrO3−δ, although different structural motifs are observed for CaCrO3−δ. The Sr1−xCaxCrO3−δ phase diagram shows that the lower temperature for superstructure formation has a maximum near x = 0.5, evidencing an influence of Ca/Sr cation disorder on oxygen vacancy order.