A neutron diffraction investigation has been performed of the structure of four chalcogenide glasses, using the D4c diffractometer on the high-flux reactor at the Institut Laue-Langevin (ILL). Vitreous Ge 3As 52S 45 is shown to have a structure based on As 4S 3 molecules, whereas that of its selenide analogue, vitreous Ge 3As 52Se 45, involves far fewer As 4Se 3 molecules. Two As 2X 3 reference glasses (X = S or Se) have purely network structures. As–X, and As–As bond lengths have been extracted, together with their associated co-ordination numbers, from peak fits to the real space correlation functions, T( r), and suggest that all four glasses are chemically ordered; i.e. that they contain the maximum possible number of As–X (and Ge–X) bonds. The molecular nature of vitreous Ge 3As 52S 45 is evident from the enhanced first diffraction peak relative to the As 2S 3 reference glass, and has been further investigated by comparing its interference function, Qi( Q), with that expected for an isolated As 4S 3 molecule. On the other hand, the second diffraction peak for vitreous Ge 3As 52S 45 is not reproduced by a random orientation molecular model, indicating that there is orientational correlation between adjacent molecules, and this is discussed with respect to the structure of the corresponding As 4S 3 crystalline phases. The neutron-weighted vibrational density of states (VDOS) for vitreous Ge 3As 52S 45, obtained with the ISIS MARI spectrometer, unambiguously confirms the existence of As 4S 3 molecules, as revealed by a peak at ~ 34 meV arising from the triangle of As atoms that form the base of the molecule. MARI data recorded at ambient temperature also reveal a strong quasi-elastic component in the scattered intensity, which has been further investigated using the IN5 quasi-elastic scattering spectrometer (ILL) and indicates the presence of rotational diffusion of the As 4S 3 molecules; i.e. that, at ambient temperature, vitreous Ge 3As 52S 45 behaves as a plastic glass. A possible structural model for vitreous Ge 3As 52S 45 comprises a two-dimensional tangled GeS 2 net with As 4S 3 molecules trapped in its folds whereas, for vitreous Ge 3As 52Se 45, a clathrate model appears more appropriate.