We report the first Fourier transform infrared spectroscopic studies of the hydrogen-bonding interactions in supercritical methanol and methanol/methanol-d1 mixtures. At 523 K and below similar to0.3 mol/l, methanol chiefly existed in a monomeric form. However, at 523 K and between similar to0.3 and 2 mol/l, in the regions of O-H, C-O, and C-H stretching vibrations, there were clear indications of methanol monomer-dimer equilibrium. The effect of temperature on the equilibrium constant was also investigated. As a result, the partial molar volume difference and enthalpy of the dimerization were estimated and correlated with the behavior of the methanol isothermal compressibility. It was also concluded that at 523 K and density higher than similar to15 mol/l, the hydrogen-bonding equilibrium started to shift toward larger aggregates.