  A phase transition of gas-liquid type with an upper critical point is
examined which arises in a model of charges of one sign on compensating
background (OCP). The phase transition parameters are dependent on the detailed
assumptions about the compressibility of the background, but the occurrence of
this transition is independent on the background equation of state. In the
electron-gas model ('jellium'), this transition appears to rule out Wigner
crystallization. A variational principle in statistical mechanics is used to
derive so-called Double-OCP model for a superposition of two one-component
plasma models for charges of opposite sign. The free energy of this model sets
an upper bound to that of a real plasma. Situations are discussed where this
transition should manifest itself in anomalies in the approximate description
of a non-ideal plasma.
