Inelastic X-ray scattering experiments have been performed on methanol as a function of density from ambient to the supercritical state. Positive dispersion of the sound velocity, as compared to the hydrodynamic values, is similar to 50% in the ambient condition and decreases to zero at 0.50 g cm(-3) over the momentum transfer Q = 1-10 nm(-1) with lowering density; however, it increases again with a further decrease in density down to 0.20 g cm(-3) in the supercritical state only in the Q-range above similar to 5 nm(-1). These results have been interpreted as the formation of small oligomers in the low-density supercritical methanol. (C) 2007 Elsevier B.V. All rights reserved.