The dimerization of acrylic and of methacrylic acid in dilute solutions of supercritical ethylene was investigated via FTIR-spectroscopy in the region of the C=O stretching fundamentals. Concentrations of monomers and of hydrogen bonded dimers at temperatures up to 175 degrees C and pressures to 2500 bar were derived by deconvoluting the measured absorbance spectra with a band simulation program. For methacrylic acid, the carbonyl absorption spectra were decomposed into a monomer and a dimer component from which the pressure-dependent enthalpy of dimerization Delta H-0 = -(46.7 - 2.49 . p/kbar) kJ mol(-1) and the temperature-dependent dimerization volume Delta V-0 = (5.7 - 0.069 . T/degrees C) cm(3) mol(-1) were calculated. For acrylic acid additional absorption components occur as a consequence of s-cis-/s-trans-isomerism. The pressure-dependent enthalpy of dimerization is found to be: Delta H-0 = -(44.5 -1.85 p/kbar) kJ mol(-1).