By cooling the target chamber of a photoelectron spectrometer, the photoelectron spectra of dimers and mixed complexes of carboxylic acids have been obtained. By comparison of the spectra with each other and with those of the monomers, it has been possible to assign several bands to ionization from specific molecular orbitals. Hydrogen-bonding changes most ionization potentials of the monomers but not by more than about 0.5 eV. In particular, the ionization potential of an electron from a non-bonding orbital on the proton donor group is decreased and that from a non-bonding orbital on the electron donor group is increased. This is consistent with most theories of hydrogen bonding. In the mixed complexes it makes the first ionization potential higher than that of the free acids. dimers, however, are complicated by the initial degeneracy of their orbitals and this results in their first ionization potential being lower than for the free acid. There is also some evidence that the hydrogen bonds in a complex between different acids are of unequal strength.