A study of the effect of large-amplitude vibrations on the thermodynamic properties of malondialdehyde is presented. By using ab initio methodology at the MP2(FC)/6-311G(d,p) level, the internal rotation of the two aldehydic groups is analyzed. Two maxima are localized on the two-dimensional potential surface, and the global minimum is found in a C-1 conformation. The highest maximum appears because of steric hindrance between the oxygens, whereas the second maximum is due to steric hindrance between the hydrogens of the two aldehydic groups. After determination of the nonrigid group of the molecule, we obtain kinetic and potential functions adapted to the a(1) representation. With these functions, the two-dimensional vibrational Hamiltonian is solved variationally, and the torsional energy levels are calculated for the first time. The fundamental frequencies of vibration for the two torsional modes are found to be 48.35 and 87.01 cm(-1). The effect of nonrigidity in the thermodynamic properties of malondialdehyde is determined by comparison of nonrigid and harmonic results. In the nonrigid case, it is found that the increase of the density of states affects mainly the value of the partition function, whereas its temperature derivative is almost constant.