The crystal structure of the title compound, [CaL(H2O)3][NO3]·2H2O (L = purpurate), has been determined at 295 K by X-ray diffraction and refined by least squares to R 0.050 (1 545 ‘observed’ reflections). Crystals are monoclinic, space group Pn, a= 11.282(5), b= 9.111(3), c= 9.045(4)Å, β= 105.70(3)°Z= 2. Co-ordination about the calcium is approximately eight-co-ordinate dodecahedral. One of the trapezoidal planes comprises the usual tridentate purpurate site [Ca–O, 2.567(4), 2.429(4); Ca–N, 2.600(5)Å] and a further purpurate bridging oxygen [Ca–O, 2.361(5)Å] while the other comprises the three co-ordinated water molecules [Ca–O, 2.372(7), 2.378(5), 2.460(5)Å] and another bridging purpurate oxygen [Ca–O, 2.548(6)Å], an infinite polymer resulting parallel to bc. The geometry within the nitrate isvery asymmetric [N–O, 1.213(10)–1.282(14)Å, O–N–O, 115.7(8)–122.9(8)°], and is found to correlate closely with the observed hydrogen bonding about the nitrate.