X-ray powder diffraction data for the two new polymorphs of 1-methylhydantoin, C4H6N2O2, are reported. The polymorph II (MH-II) crystallizes in the orthorhombic system with space group Pna21 [a = 19.0323(7) Å, b = 3.91269(8) Å, c = 6.8311(7) Å, Z′ = 1, Z = 4, unit cell volume V = 508.70(3) Å3. Polymorph III (MH-III) crystallizes in the orthorhombic system with space group P212121 [a = a = 7.82427(5), b = 9.8230(5), c = 20.2951(4), Z′ = 3, Z = 12, unit cell volume V = 1563.5(1) Å3]. All measured lines, in each case, were indexed and are consistent with the space group.
