Optical applications of chalcogenide glasses need a wide transparency window in far IR. Promising glass systems include HgI2. We have synthesised three vitreous systems yHgI2.(1-y)As2X3, where X = S, Se, Te. The max HgI2 content in the glass increases with atomic number from y = 0.2 (S) to y = 0.8 (Te). In order to decode the structure of these exciting new glasses and to understand an enormous difference in their glass-forming ability, we are planning ND measurements of 8 glasses in the 3 systems. Our recent feasibility experiment using GEM has shown that we are able to obtain a high-quality data on these strongly absorbing samples. In particular, the y = 0.2 sulphide glass seems to contain mercury iodide in a molecular form. This is the first direct structural information on mercury environment in a chalcohalide glass.