In 2010 several groups showed that iron selenide, already a superconductor at 8 K, could be charged with alkali cations leading to higher critical temperatures. These compounds consist of the general formula A2Fe4Se5 where A = K, Rb, Cs, or Tl. We have prepared a related phase KFe2Se3, which undergoes an antiferromagnetic transition at 130 K as evidenced by our SQUID magnetization data. Instead of 2D sheets of edge-sharing FeSe4 tetrahedra as in the 245-phases, however, the dimensionality has been reduced to a double chain structure in the 123-phase. We propose a neutron diffraction experiment using the Wish diffractometer at ISIS to study the magnetic and nuclear structure of KFe2Se3 as a function of temperature and K occupancy. This study aims to establish the crystal structure above and below 130 K, magnetic ordering, size of the iron moment, and chemical stoichiometry.