Experimental results of multiphase fluid equilibria occurring in ternary mixtures of near-critical carbon dioxide + dodecane + o-nitrophenol and near-critical carbon dioxide + octanol + o-nitrophenol are presented. According to the classification of Van Konynenburg and Scott, the binary subsystems of carbon dioxide with dodecane and octanol show type-II and type-III fluid phase behavior respectively, whereas the binary subsystem of carbon dioxide with o-nitrophenol shows type-V fluid phase behavior. Starting with one of the binary subsystems with its characteristic type of fluid phase behavior, the transformation into the fluid phase behavior of the second binary subsystem can be investigated by gradually exchanging the first solute with the second. This study focuses on the transformations of type-V fluid phase behavior into type-II or type-III respectively. Surprisingly, from this study it became apparent that there is a strong indication that the binary subsystem carbon dioxide + o-nitrophenol does not have the accepted type-V fluid phase behavior but type-IV. Additional research is underway to prove that the latter phenomenon applies in general.