The critical line of a nearly ideal binary system generally moves from the critical point of one of the components directly to the critical point of the other component (curve 1 Fig. 1). In a less ideal system, however, the behaviour is quite different. In some cases the curves move from the critical point of the less volatile component (component 2) to lower temperatures and higher pressures (curve 2) and rise again to higher temperatures via a temperature minimum, the critical double point. In other systems, the critical temperature increases continuously from the critical point of the less volatile component when the pressure is increased (curve 3). We assume here that the critical line is not interrupted by the appearance of a solid phase.