The anhydrous and hydrated lithium monoborates have been studied. The most hydrated phase is LiBO2·8H2O; its structural formula in the P3 space group is Li(H2O)4B(OH)4·2H2O. Refinement of the cell parameters yielded the following results: a=6.5483(5) Å, c=6.1692(7) Å with F(30)=64(0.015, 32), Z=1, and Dx=1.402 g/cm3. This phase gives LiB(OH)4 by spontaneous dehydration. An X-ray powder diffraction study of LiB(OH)4 as a function of temperature indicated three poorly crystallized hydrates. Two of these hydrates have the formula LiBO2·0.3H2O; the other, LiBO2·xH2O, has an undetermined water content. Crystal data for α-LiBO2 have been obtained: a=5.8473(10) Å, b=4.3513(6) Å, c=6.4557(10) Å, β=115.08(1)°, F(27)=58.5(0.001, 41); space group P21/c, Z=4, and Dx=2.18 g/cm3. β-LiBO2 does not exist but corresponds to the α-LiBO2 form observed at 600 °C. Numerous other LiBO2 forms reported recently have not been found.