A synchrotron x-ray powder-diffraction study has been carried out on two samples with well-defined excess oxygen concentrations, δ = 0.045 and 0.055. Evidence for one-dimensional interstitial oxygen ordering along the c -axis was observed from the appearance of a pair of superlattice satellite peaks at (0, 1,2 ± Δ) in both samples. In La2CuO4.045, a ∼5% oxygen-poor Bmab phase coexists with a stage-6 phase and with a small amount of the high-temperature-tetragonal (HTT) phase below the phase separation temperature (∼280 K). La2CuO4.055 shows a single stage-5 phase below 170 K, but with two unindexed peaks between (0, 1, 2 ± 0.2) satellites which may well result from a further domain modulation. The critical temperature Tc is increased by 1.5 K by reducing the stage number from 6 to 5.