The crystal structure of hexagonal α-LiIO3 has been determined by neutron and x-ray diffraction at ten temperatures from 20 to 500 K. Four sets (1248 to 1622 reflections per set) of neutron data at λ=1.0467 Å were measured at the Brookhaven High Flux Beam Reactor at 20, 100, 200, and 295 K. The integrated intensities of a further 13 data sets (1706 to 2963 reflections per set) were measured using MoKα radiation on a CAD-4 x-ray diffractometer. The structure at each temperature was refined by least squares in space group P63. The final agreement factors fell between 0.023 and 0.050. The unit cell dimensions at 295 K are a=5.481 27(6), c=5.171 65(4) Å, with nonlinear thermal expansions. The principal change in structure with temperature is in the atomic positions of Li and O relative to I: The positions are given by O(z)T=−0.8435(3) Å+62(4)×10−9 T2 Å K−2 and Li(z)T=0.3808(10) Å+167 (18)×10−9 T2 Å K−2. The iodate ion has a constant I–O bond length of 1.8081(4) Å, but the O–I–O angle increases from 99.97(2)° at 20 K by 0.5° over the temperature range studied. The LiO6 octahedron has Li–O distances of 2.070 and 2.140(1) Å at 20 K, increasing to 2.106 and 2.151(9) Å at 460 K. The mean-square amplitude of thermal vibration varies nonlinearly with temperature for all atoms, with zero-point rms displacement of 0.040 Å for I, 0.067 Å for O, and 0.094 Å for Li. The Debye temperature is estimated as 350 K on the basis of the 295 K amplitudes of vibration.