Premixed low-pressure tetrahydrofuran/oxygen/argon flames are investigated by photoionization molecular-beam mass spectrometry using vacuum-ultraviolet synchrotron radiation. For two equivalence ratios (φ = 1.00 and 1.75), mole fractions are measured as a function of distance from the burner for almost 60 intermediates with molar masses ranging from 2 (H2) to 88 (C4H6O2), providing a broad database for flame modeling studies. The isomeric composition is resolved by comparisons between experimental photoionization efficiency data and theoretical simulations, based on calculated ionization energies and Franck-Condon factors. Special emphasis is put on the resolution of the first reaction steps in the fuel destruction. The photoionization experiments are complemented by electron-ionization molecular-beam mass-spectrometry measurements that provide data with high mass resolution. For three additional flames with intermediate equivalence ratios (φ = 1.20, 1.40 and 1.60), mole fractions of major species and photoionization efficiency spectra of intermediate species are reported, extending the database for the development of chemical kinetic models.