at 10 GPa*

We report measurements of electrical resistivity as a function of temperature, at pressures up to 20 GPa, on the layered BiS₂-based superconducting system LnO_0.5F_0.5BiS₂ (Ln = La, Ce). Under pressure, compounds with Ln = La, Ce, Pr, Nd or Yb exhibit a rapid and pronounced increase in the superconducting transition temperature (T_c) from a low-T_c phase to a higher-T_c phase, followed by a gradual decrease in T_c as pressure is increased. LaO_0.5F_0.5BiS₂ shows an enhancement of T_c from 3.3 K at 0 GPa to 10.1 K at 1.1 GPa followed by a monotonic decrease up to 15 GPa. CeO_0.5F_0.5BiS₂ displays an increase of T_c from 2.3 K at 0 GPa to 6.7 K at 2 GPa. Preliminary results indicate a subsequent enhancement of T_c from 4.7 K at 5.5 GPa to 7.2 K at 10 GPa. This may be associated with a pressure-induced increase in the valence of Ce that electron dopes the BiS₂ planes. This is consistent with a previous report on the effect of substituting tetravalent Th, Hf, Zr, or Ti for trivalent La in inducing superconductivity in LaOBiS₂. We are performing further measurements to substantiate these results.