  Details of the Fermi-surface topology of deuterated
$\kappa$-(BEDT-TTF)$_2$Cu(NCS)$_2$ ~have been measured as a function of
pressure, and compared with equivalent measurements of the undeuterated salt.
We find that the superconducting transition temperature is much more
dramatically suppressed by increasing pressure in the deuterated salt. It is
suggested that this is linked to pressure-induced changes in the Fermi-surface
topology, which occur more rapidly in the deuterated salt than in the
undeuterated salt as the pressure is raised. Our data suggest that the negative
isotope effect observed on deuteration is due to small differences in
Fermi-surface topology caused by the isotopic substitution.
