The vapor pressure measurement unit from borosilicate glass was assembled for determination of partial pressures of water in aggressive acidic media using the transpiration method. Three ternary systems H2OHNO3RE(NO3)(3) (RE = Pr, Nd, Sm) were investigated. The experimental results in ternary systems were compared with the estimation by the Pitzer model and Zdanovsky rule from the data of binary subsystems. In addition to vapor pressure measurements densities of investigated solutions were determined.