[en] This paper examines a phase transition of gas-liquid type with an upper critical point which arises in a model for charges of one sign against a compensating background. The transition parameters are dependent on the detailed assumptions about the compressibility of the background; the occurrence of the transition is independent of approximate calculations on the equation of state. In the electron-gas (gel) model, this transition appears to rule out Wigner crystallization. A variational principle in statistical mechanics is used to derive what is called a duplicated model for a one-component plasma, whose free energy sets an upper bound to that of a real plasma. Situations are discussed where this transition should make itself felt in anomalies in the approximate description of a non-ideal plasma