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, c II = 11.0203(17) Å. The subcells combine into a coincidence (super)cell with b = 92.79(1) Å, which is 25bI and 34b II. The structure of one of the subcells appears to be related to that of Ca0.82CuO2 and NaCuO2. Sr 0.74CuO2 is non-superconducting, at least above 4 K.