The pressure of selenium was measured by the static method with quartz membrane-gauge manometers in the systems LnSe(2)-LnSe(1.5) (Ln = Ce, Pr) within the temperature range (893-1332) K. From p(Se)-T-x dependences obtained, the composition of intermediate phases and the thermodynamic characteristics (Delta H-r(298), Delta S-r degrees(298)) for the stepwise dissociation processes in the systems studied were calculated. Data obtained in our previous works for the systems LnSe(2)-LnSe(1.5) (Ln = La, Pr, Nd) were reprocessed in connection with more correct estimation of heat capacities and absolute entropies of polyselenides. The standard thermodynamic functions (Delta H-f(298), S degrees(298), Delta(f)G degrees(298)) of La, Ce, Pr, Nd polyselenides were determined. Solid phase compositions in the systems LnSe(2)-LnSe(1.5) (Ln = La, Ce, Pr, Nd) may be described by general formula Ln(n)Se(2n-1) (n = 5, 7, 10, 20). The dependence of thermodynamic values of polyselenides on the selenium content was analysed. The set of thermodynamic characteristics obtained may be used for phase equilibria calculation and for thermodynamic modelling of single-crystal growth processes in the systems LnSe(2)-LnSe(1.5) (Ln = La, Ce, Pr, Nd). (C) 2012 Elsevier Ltd. All rights reserved.