The average structure in the space group C2/m at 130 K of strontium pentacyanonitrosylferrate(II) tetrahydrate (strontium nitroprusside tetrahydrate, Sr[Fe(CN) 5 NO].4H 2 O, space group P2/m, monoclinic, Z=4, a=19.816(24), b=7.563(7), c=8.406(9) 9, β=99.02 o , V=1244(24) 9 3 ) has been determined using neutron diffraction. A final R factor 0.061 was obtained using 665 observed structure factors.When going from room temperature to 130 K the space group C2/m changes to P2/m. A rearrangement of hydrogen atoms is produced due to 14.3, 17.2, 5.9 and 26 o rotations of the planes' of W1, W2, W3 and W4 water molecules, respectively. As a consequence of this structural modification, the weakening of some hydrogen bonds related to the W4 molecule and the reinforcement of the hydrogen bonds of the W2 molecule along the [0 1 0] direction are observed.