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 °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 ΔHo = −(46.7 − 2.49 • p/kbar) kJ mol−1 and the temperature-dependent dimerization volume ΔVo = (5.7 − 0.069 • T/°C) cm3 mol-1 were calculated. For acrylic acid additional absorption components occur as a consequence of s-cis-/s-trans-iso-merism. The pressure-dependent enthalpy of dimerization is found to be: ΔHo = −(44.5 − 1.85 p/kbar) kJ mol-1.