The structure of GeSe4 glass will be measured using the method of isotope substitution in neutron diffraction with both Ge and Se isotopes. This material is representative of a family of network forming glasses for which there is a transition in their physical properties around a mean coordination number n = 2.4. The results will yield unprecedented information on the structure of GeSe4 glass to help distinguish between the various models that have been proposed. These range from a chemically ordered network based on undefective GeSe4 corner-sharing tetrahedra, in which all Ge atoms are connected by Se2 dimers such that the fraction of Ge-Se-Se connections is 100%, to a phase separated model in which the fraction of Ge-Se-Se connections is zero. The diffraction results will be complemented by 77Se MAS NMR experiments and first principles molecular dynamics simulations.