Kinetics and mechanism of phase formation in the Ln-O-S (Ln=La, Gd, Y) systems during Ln(2)O(3) sulfidation in ammonium thiocyanate vapor and the identified sequence of transformations of obtained phases in the temperature range 873-1273K have been studied. The kinetic dependencies for the reactions involved were fitted by the Jander equationfor topochemical heterogeneous reactions with a squared correlation coefficient R-2=.99. The reaction rate constants, pre-exponential factors, and effective activation energies for the reaction formation of compounds have been calculated, and approximate times and low temperatures for the formation of particular phases have been found.