Raman spectra of mixtures of acetic acid with donor solvents not having C=O groups have been studied. The solvents chosen are dioxan and acetal. The results lend definite support to the mechanism described in previous papers (Parts III² and IV¹) of the progressive breakdown of the ring dimers of the acid into associates of the open dimers and eventually of the monomers with the solvent molecules. The extraordinary efficiency of water in bringing about the breakdown, is attributed to its dual capacity to act as both donor and acceptor.