  We report high pressure X-ray diffraction and a detailed systematic Raman
measurements on $ReS_2$ sample, which is mechanically exfoliated from a single
crystal. A few new Bragg peaks are observed to emerge above 6 GPa indicating a
structural transition from distorted $1T$ to distorted $1T$$^{\prime}$ in
triclinic structure. The same is corroborated by appearance of new Raman modes
in the same pressure range. Softening of the Raman modes corresponding to $Re$
atom vibrations are observed in the distorted $1T$$^{\prime}$ phase in the
pressure range 15-25 GPa. In the same pressure range the anomalous change in
the volume is found to be induced by the lattice expansion. The volume
expansion is related to the sliding of layers leading to octahedral distortion
and increase in octahedral volume. The sample is found to be much
incompressible above 25 GPa with respect to below 15 GPa data. The same is also
reflected in the Raman mode shifts with pressure.
