Magnetization and high resolution neutron powder diffraction measurements on the magnetic shape memory compound Ni2Mn1.44Sn 0.56 have confirmed that it is ferromagnetic below 319 K and undergoes a structural phase transition which takes place at TM ≤ 221 K on cooling and 239 K on warming. The high temperature phase has the cubic L 21 structure, a ≤ 5.973 Å, with the excess manganese atoms occupying the 4(b) tin sites. In the cubic phase at 245 K the manganese moments at both sites were found to be ferromagnetically aligned. The magnetic moment at the 4(a) sites was 1.88(10) μB but it was only 0.53(18) μB/Mn at the 4(b) sites. The low temperature phase stable below TM has an orthorhombic structure with space group Pmma related to the cubic phase through a Bain transformation aortho ≤ (a cub+bcub)/2; bortho ≤ ccub; cortho ≤ (acub- bcub). The change in cell volume in the transition is only ≈0.5%, suggesting that the atomic moments are unchanged although the spontaneous magnetization drops significantly.