Phase diagrams predicted by the Redlich-Kwong equation of state (EOS) and classical mixing rules are discussed at the transitional states between different types of phase behavior. Mixtures containing molecules of different sizes have been considered in the analysis. Zero- and infinite-pressure reference states for the estimation of the Gibbs energy of mixing are presented. These reference states constitute hypothetical states that have been found valuable for analyzing the key features of critical lines ending at these limiting conditions. An expression for predicting the possibility of critical immiscibility at infinite pressure was developed. This expression can be useful for determination of the existence of liquid-liquid critical lines in binary systems and may be applied to every cubic EOS of van der Waals type when classical mixing rules are used. The zero-pressure approach was used for analyzing the existence of the critical lines that intersect the zero-pressure line in a P-T projection. Special attention has been given to the possibility of predicting closed loops of liquid-liquid immiscibility. Closed loops were detected at the transition from Type V to Type III, but they do not correspond to the traditional topology observed in systems of Type VI and VII. The relation of the calculated diagrams to the systems with real pure component critical properties showing closed loops of liquid-liquid immiscibility is also discussed.