All the 12 elasto-optic constants of barite have been determined and the values obtained for λ 5893 are $$\begin{gathered} p_{11} = 0 \cdot 21 p_{44} = 0 \cdot 002 p_{12} = 0 \cdot 25 p_{23} = 0 \cdot 19 \hfill \\ p_{22} = 0 \cdot 24 p_{55} = 0 \cdot 012 p_{13} = 0 \cdot 16 p_{31} = 0 \cdot 275 \hfill \\ p_{33} = 0 \cdot 31 p_{66} = 0 \cdot 037 p_{21} = 0 \cdot 34 p_{32} = 0 \cdot 22 \hfill \\ \end{gathered} $$ The behaviour of the optical properties of barite on uniform compression on all sides is discussed. The true temperature coefficients of the three principal refractive indices have also been calculated.