A mean-field theory of phase separation in liquid-metal alloys is described. The method satisfactorily accounts for the variation of critical concentration in alloys of simple metals, and also gives critical temperatures in reasonably good agreement with experiment. It predicts that under pressure the critical temperature should rise proportionally to the ionic number density. The theory leads to typical mean-field divergences in the x-ray scattering intensity, specific heat, and temperature derivative of the resistivity. The relation between the instability that causes such divergences and the "freezing instability" described by Schneider et al. is discussed.