The strontium oxide–copper oxide binary phase diagram has been reinvestigated under oxygen pressures of up to 600 bar, in the temperature range 600–850 °C. A new, high-pressure phase, Sr0.74CuO2, has been isolated and characterised. The oxygen stoichiometry, confirmed by thermogravimetry, gives an average formal copper valence of ca. 2.5. X-Ray and electron diffraction analyses suggest that Sr0.74CuO2 has a Vernier structure incommensurate in b. The subcells are body-centred orthorhombic: aI= 6.8182(12)Å, bI= 3.7113(7)Å, cI= 11.0203(17)Å, and face-centred orthorhombic: aII= 6.8182(12)Å, bII= 2.7290(3)Å, cII= 11.0203(17)Å. The subcells combine into a coincidence (super)cell with b= 92.79(1)Å, which is 25bI and 34bII. The structure of one of the subcells appears to be related to that of Ca0.82CuO2 and NaCuO2. Sr0.74CuO2 is non-superconducting, at least above 4 K.