Abstract The IR (4000–400 cm −1 ) and Raman (4000–50 cm −1 ) spectra of 3-methoxymethylene-2,4-pentanedione (H 3 C–O–CH C(COCH 3 ) 2 ) in the liquid and solute forms in various solvents of different polarity were recorded at ambient temperature. Additional IR and Raman spectra were obtained for amorphous and crystalline solid at low temperature. The vibrational spectra revealed that compound exists atleast in two dominant conformers with different polarity and that conformer present in the solid phase is less polar. NMR spectra in various solvents at different temperatures were also obtained.The compound can exist in several conformers as a result of rotation around O–C and both C–C bonds with planar or nonplanar arrangement of the heavy atoms. Semiempirical (AM1, PM3, MNDO, MINDO3) and ab initio using TZP and 6-31G** basis sets calculations were carried out. According to ab initio calculations at least five conformational structures with the methoxy group oriented as anti or syn and carbonyl groups oriented as Z or E towards the C C double bond were obtained at energy surface. The calculated ab initio and AM1 energies of all conformers suggest as the most stable anti-ZE conformer where Z and E regard of the trans and cis acetyl group, respectively. As the second most stable conformer with energy at least 10 kJ mol −1 higher was calculated anti-EZ conformer. Assignments of the vibrational spectra for the studied compound were made with the aid of normal coordinate calculations employing scaled ab initio force field constants. The scaled ab initio frequencies as well as calculated energies indicate that the conformer present in the solid phase is anti-ZE .