This work provides new temperature-dependent mole fractions of elusive intermediates relevant to the lowtemperature oxidation of dimethyl ether (DME). It extends the previous study of Moshammer et al. [J. Phys. Chem. A 2015, 119, 7361−7374] in which a combination of a jet-stirred reactor and molecular beam mass spectrometry with singlephoton ionization via tunable synchrotron-generated vacuumultraviolet radiation was used to identify (but not quantify) several highly oxygenated species. Here, temperature-dependent concentration profiles of 17 components were determined in the range of 450−1000 K and compared to up-to-date kinetic modeling results. Special emphasis is paid toward the validation and application of a theoretical method for predicting photoionization cross sections that are hard to obtain experimentally but essential to turn mass spectral data into mole fraction profiles. The presented approach enabled the quantification of the hydroperoxymethyl formate (HOOCH 2 OCH 2 O), which is a key intermediate in the low-temperature oxidation of DME. The quantification of this ketohydroperoxide together with the temperature-dependent concentration profiles of other intermediates including H 2 O 2 , HCOOH, CH 3 OCHO, and CH 3 OOH reveals new opportunities for the development of a next-generation DME combustion chemistry mechanism.