Wide angle neutron diffraction in combination with H/D substitution was used to determine the inter- and intramolecular structure of 100% phosphoric acid (H3PO4, PA). From radial distribution functions g(HH)(r), g(HX)(r), and g(XX)(r) (where X is either O or P) the hydrogen bonds were found to be characterized by a very short O...H distance (1.54 Angstrom). Within a molecule, the orientation of an OH group was found to be preferably in one of the three O-P-O planes. In the interpretation of the radial distribution functions, use was made of preliminary results of molecular dynamics simulations. Temperature effects on the structure of PA were only found in the hydrogen bond structure, which becomes somewhat less well defined when heating up from room temperature to 60 degrees C. Polyphosphates could not be detected, probably due to the small degree of polymerization.