The structural, vibrational, andphotochemical study of 1-methylhydantoin(1-MH, C4H6N2O2) was undertakenby matrix isolation infrared spectroscopy (in argon matrix; 10 K),complemented by quantum chemical calculations performed at the DFT(B3LYP)/6-311++G(d,p)level of approximation. The theoretical calculations yielded the Cssymmetry structure, with planar heavy atomskeleton, as the minimum energy structure on the potential energysurface of the molecule. The electronic structure of this minimumenergy structure of 1-MH was then studied in detail by means of thenatural bond orbital (NBO) and atoms in molecules (AIM) approaches,allowing for the elucidation of specific characteristics of the molecule’sσ and π electronic systems. The infrared spectrum of thematrix-isolated 1-MH was fully assigned, also with the help of thetheoretically predicted spectrum of the compound, and its UV-inducedunimolecular photochemistry (λ ≥ 230 nm) was investigated.The compound was found to fragment to CO, isocyanic acid, methylenimine,and N-methyl–methylenimine. Finally, a thermalbehavior investigation on 1-MH samples was carried out using infraredspectroscopy (10 K until melting), differential scanning calorimetryand polarized light thermal microscopy. A new polymorph of 1-MH wasidentified. The IR spectra of the different observed phases were recordedand interpreted. [ABSTRACT FROM AUTHOR]