Results of an exhaustive experimental study of the valence electronic structure of thiophene using high resolution electron momentum spectroscopy at impact energies of 1200 and 2400 eV are presented. The measurements were performed using an electron momentum spectrometer of the third generation at Tsinghua University, which enables energy, polar and azimuthal angular resolutions of the order of Delta E = 0.8 eV, 00 = 0.53 and Delta phi = +/- 0.84 degrees. These measurements were interpreted by comparison with Green's function calculations of one-electron and shake-up ionization energies as well as of the related Dyson orbital electron momentum distributions, using the so-called third-order algebraic diagrammatic construction scheme (ADC(3)). Comparison of spherically averaged theoretical electron momentum distributions with experimental results very convincingly confirms the presence of two rather intense pi(-2) pi*(+1) shake-up lines at electron binding energies of 13.8 and 15.5 eV, with pole strengths equal to 0.18 and 0.13, respectively. Analysis of the electron momentum distributions associated with the two lowest (2)A(2) (pi(-1)(3)) and B-2(1) (pi(-1)(2)) cationic states provides indirect evidence for a, symmetry lowering and nuclear dynamical effects due to vibronic coupling interactions between these two states. ADC(3) Dyson orbital momentum distributions are, systematically compared with distributions derived from Kohn-Sham (B3LYP) orbitals, and found to provide most generally superior insights into experiment.