Features of water fluids at increased T and P are defined by presence or absence of critical phenomena in saturated solutions. The 1st type includes solutions with compounds (NaCl, KCl, CaCl2, MgCl2, LiCl, NaOH, KOH, KF, H3BO3 and others) demonstrating a positive solubility at temperature increasing. In the 2nd type the solubility of compounds (NaF, LiF, Na2CO3, Na2SO4, K2SO4, Li2SO4, SiO2, silicate minerals) reaches the minimum values at P point (close to the critical point H2O), while at high T (above Q point) the solubility considerably increases. It is important to define a type of solutions in fluid inclusions (FI). An incorrect rating of PQ type solutions, for example, to chloride system leads to considerable overestimating of the calculated pressure. It is rather complicated to predict properties of natural fluids using only the data of thermometric studies of FI without additional information about its composition. Nevertheless, there are the characteristic features specifying type of solutions in FI. For instance, gradual dissolution of daughter crystals at rising T, joint occurrence of gaseous and liquid FI with soluble phases unequivocally characterize the 1st type of solutions. Occurrence of daughter phases which are not dissolved at heating, absence of opening features of essentially gaseous FI up to very high T (1200-1400°C) indicates that the solutions in FI belong to PQ type. FI with low-density water-salt solutions can be also used for study of the upper area of immiscibility existence in PQ type systems.