Pressure induced structural transition and elastic properties of ZnS-type (B3) to NaCl-type (B1) structure in mercury chalcogenides (HgX; X = S. Se and Te) are presented. An effective interionic interaction potential (EIOP) with long-range Coulomb, as well charge transfer interactions. Hafemeister and Flygare type short-range overlap repulsion extended up to the second neighbor ions and van der Waals interactions are considered. Emphasis is on the evaluation of the pressure dependent Poisson's ratio v, the ratio R-BT/G of B (bulk modulus) over G (shear modulus). anisotropy parameter. Shear and Young's modulus, Lame constant, Kleinman parameter, elastic wave velocity and thermodynamical property as Debye temperature. The Poisson's ratio behavior infers that Mercury chalcogenides are brittle in nature. To our knowledge this is the first quantitative theoretical prediction of the pressure dependence of elastic and thermodynamical properties explicitly the ductile (brittle) nature of HgX and still awaits experimental confirmations. (C) 2012 Elsevier B.V. All rights reserved.