NaHSeO3 crystallizes in the monoclinic C2/c space group. There are two inequivalent Na sites. In the first Na site, Na(1) is bonded to one O(1), one O(2), one O(3), one O(4), one O(5), and one O(6) atom to form a mixture of corner and edge-sharing NaO6 octahedra. In the second Na site, Na(2) is bonded to one O(1), one O(4), one O(5), and two equivalent O(3) atoms to form distorted NaO5 trigonal bipyramids that share corners with three equivalent Na(1)O6 octahedra, corners with two equivalent Na(2)O5 trigonal bipyramids, and  an edgeedge with one Na(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 62-80°. There are two inequivalent H sites. In the first H site, H(1) is bonded in a linear geometry to one O(1) and one O(4) atom. In the second H site, H(2) is bonded in a distorted single-bond geometry to one O(2) and one O(6) atom. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a trigonal non-coplanar geometry to one O(2), one O(5), and one O(6) atom. In the second Se site, Se(2) is bonded in a trigonal non-coplanar geometry to one O(1), one O(3), and one O(4) atom. There are six inequivalent O sites. In the first O site, O(5) is bonded in a trigonal planar geometry to one Na(1), one Na(2), and one Se(1) atom. In the second O site, O(6) is bonded in a 3-coordinate geometry to one Na(1), one H(2), and one Se(1) atom. In the third O site, O(1) is bonded to one Na(1), one Na(2), one H(1), and one Se(2) atom to form a mixture of distorted corner and edge-sharing ONa2HSe trigonal pyramids. In the fourth O site, O(2) is bonded in a distorted single-bond geometry to one Na(1), one H(2), and one Se(1) atom. In the fifth O site, O(3) is bonded to one Na(1), two equivalent Na(2), and one Se(2) atom to form distorted ONa3Se tetrahedra that share corners with two equivalent O(3)Na3Se tetrahedra, corners with two equivalent O(1)Na2HSe trigonal pyramids, and  an edgeedge with one O(1)Na2HSe trigonal pyramid. In the sixth O site, O(4) is bonded in a distorted single-bond geometry to one Na(1), one Na(2), one H(1), and one Se(2) atom.
NaHSeO3 crystallizes in the monoclinic C2/c space group. There are two inequivalent Na sites. In the first Na site, Na(1) is bonded to one O(1), one O(2), one O(3), one O(4), one O(5), and one O(6) atom to form a mixture of corner and edge-sharing NaO6 octahedra. The Na(1)-O(1) bond length is 2.44 Å. The Na(1)-O(2) bond length is 2.47 Å. The Na(1)-O(3) bond length is 2.50 Å. The Na(1)-O(4) bond length is 2.51 Å. The Na(1)-O(5) bond length is 2.46 Å. The Na(1)-O(6) bond length is 2.41 Å. In the second Na site, Na(2) is bonded to one O(1), one O(4), one O(5), and two equivalent O(3) atoms to form distorted NaO5 trigonal bipyramids that share corners with three equivalent Na(1)O6 octahedra, corners with two equivalent Na(2)O5 trigonal bipyramids, and  an edgeedge with one Na(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 62-80°. The Na(2)-O(1) bond length is 2.53 Å. The Na(2)-O(4) bond length is 2.69 Å. The Na(2)-O(5) bond length is 2.35 Å. There is one shorter (2.38 Å) and one longer (2.45 Å) Na(2)-O(3) bond length. There are two inequivalent H sites. In the first H site, H(1) is bonded in a linear geometry to one O(1) and one O(4) atom. The H(1)-O(1) bond length is 1.60 Å. The H(1)-O(4) bond length is 1.03 Å. In the second H site, H(2) is bonded in a distorted single-bond geometry to one O(2) and one O(6) atom. The H(2)-O(2) bond length is 1.02 Å. The H(2)-O(6) bond length is 1.70 Å. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a trigonal non-coplanar geometry to one O(2), one O(5), and one O(6) atom. The Se(1)-O(2) bond length is 1.86 Å. The Se(1)-O(5) bond length is 1.69 Å. The Se(1)-O(6) bond length is 1.70 Å. In the second Se site, Se(2) is bonded in a trigonal non-coplanar geometry to one O(1), one O(3), and one O(4) atom. The Se(2)-O(1) bond length is 1.71 Å. The Se(2)-O(3) bond length is 1.70 Å. The Se(2)-O(4) bond length is 1.84 Å. There are six inequivalent O sites. In the first O site, O(5) is bonded in a trigonal planar geometry to one Na(1), one Na(2), and one Se(1) atom. In the second O site, O(6) is bonded in a 3-coordinate geometry to one Na(1), one H(2), and one Se(1) atom. In the third O site, O(1) is bonded to one Na(1), one Na(2), one H(1), and one Se(2) atom to form a mixture of distorted corner and edge-sharing ONa2HSe trigonal pyramids. In the fourth O site, O(2) is bonded in a distorted single-bond geometry to one Na(1), one H(2), and one Se(1) atom. In the fifth O site, O(3) is bonded to one Na(1), two equivalent Na(2), and one Se(2) atom to form distorted ONa3Se tetrahedra that share corners with two equivalent O(3)Na3Se tetrahedra, corners with two equivalent O(1)Na2HSe trigonal pyramids, and  an edgeedge with one O(1)Na2HSe trigonal pyramid. In the sixth O site, O(4) is bonded in a distorted single-bond geometry to one Na(1), one Na(2), one H(1), and one Se(2) atom.