Black-brown needle-shaped single crystals of [Co2(en)4(O2)(OH)][C4O4]1.5*H2O (en = ethylenediamine) have been prepared in aqueous solution at room temperature. Space group P1 ̅ (no.2) with a = 800.20(8), b = 1225.48(7), c = 1403.84(9) pm, alpha = 100.282(5), beta = 94.515(7), gamma = 95.596(6)°. The Co3+ cations [Co(1), Co(2)] are coordinated in an octahedral manner by four nitrogen atoms stemming from the ethylenediamine molecules and two oxygen atoms each from a hydroxo group and a peroxo group, respectively. Both Co3+ coordination polyhedra are connected by a common corner and by the peroxo group leading to the binuclear [(en)2Co(O2)(OH)Co(en)2]3+ cation. The squarate dianions, not bonded to Co3+, and the [(en)2Co(O2)(OH)Co(en)2]3+ cations are linked by hydrogen bonds forming a three-dimensional supramolecular network containing water molecules. Magnetic measurements reveal a diamagnetic behaviour indicating a low-spin electron configuration of Co3+. The UV-Vis spectra shows two LMCT bands (pi*(O22-) -> d(sigma)*(Co3+)) at 274 and 368 nm and the d-d transition (1A1g -> 1T1g) at 542 nm. Thermoanalytical investigations in air show that the compound is stable up to 120 °C. Subsequent decomposition processes to cobalt oxide are finished at 460 °C.