Evaporation of the mixed-ligand complex of praseodymium diethyldithiocarbamate with o-phenanthroline [Pr(S2CNEt2)(3)(Phen)] was studied by the Knudsen effusion/mass spectrometry method. The saturated vapor was found to consist of phenanthroline and Pr(S2CNEt2)(3) molecules. The partial pressures of the gas components and the enthalpy of phenanthroline elimination were determined. The vaporization pattern established from the data of mass-spectrometry for Pr(S2CNEt2)(3) is also valid for [Ln(S2CNEt2)(3)(Phen)] (Ln = La, Ce, Pr, Nd, Sm, Eu, Er, Yb, Lu).