Poly(oxymethylene) dimethyl ethers (OME) are an interesting class of oxygenated fuel components and solvents for the absorption of carbon dioxide. The chemical structure of OME_n is H₃C–O–(CH₂O)_n–CH₃ with n = 2 and the IUPAC names are methoxy(methoxymethoxy)methane (n = 2), 2,4,6,8-tetraoxanonane (n = 3), and 2,4,6,8,10-pentaoxaundecane (n = 4). This work studies the liquid-liquid equilibrium (LLE) in the binary systems (water + methylal), (water + OME₂), (water + OME₃), and (water + OME₄) and the ternary systems (water + methanol + OME₂), (water + methanol + OME₃), (formaldehyde + water + OME₂), (formaldehyde + water + OME₃), and (water + methylal + OME₂) in the temperature range 280 K – 340 K. The systems were studied by gas chromatographic- and titrimetric analysis of samples that were drawn from the coexisting liquid phases, as well by in situ analysis with a medium-field NMR spectrometer. The LLE was modeled by extending a UNIFAC-based activity coefficient model of the system (formaldehyde + water + methanol + methylal) from the literature. One new structural group is introduced to represent the OME.