The solubility, liquid-liquid equilibria (LLE), and critical states of LLE for acetic acid-propanol-water-n-propyl acetate system were studied under isothermal conditions. Based on the experimental data, the location of the critical curve at 313.15 K was determined. The possible cases of the intersection of chemical equilibrium (CE) and binodal surfaces under the change of external conditions, e.g. temperature, are presented. The possible view of diagrams in the case of simultaneous CE and LLE are also considered in the terms of transformed composition variables. Details of the experimental study of critical states in reactive systems and some aspects of the classification of quaternary reactive systems with critical phases are also presented. This is an abstract of a paper presented at the 18th International Congress of Chemical Process Engineering (Praque, Czech Republic 8/24-28/2008).