A description is presented for the Tc pressure-dependent behaviour of Hg-12(n − 1)n superconductors that takes into account the Casimir effect by considering the CuO2 conducting layers to act as plasma sheets. The Casimir energy arises from the parallel plasma sheets (Cu–O planes) whenever these are separated by distances on the nanometer scale. The Tc pressure dependence in Hg-12(n − 1)n superconductors can be described by a pressure-induced charge transfer together with intrinsic terms that are associated with the Casimir energy. For Hg-12(n − 1)n samples with optimum oxygen content, the charge transfer term vanishes, leaving an explicit expression for the intrinsic term. By picking up realistic parameters and inserting them in the expression for the Tc pressure dependence, we have a good agreement with experimental intrinsic term data observed in the mercury family Hg-12(n − 1)n superconductors.