By means of a tensimetric flow method and a static method with a silica-membrane zero gauge, the dependence of vapour pressure on temperature was obtained for tris(2,4-pentanedionato)ruthenium(III), Ru(aa)3, and tris(1,1,1-trifluoropentane-2,4-dionato)ruthenium(III), Ru(tfa)3. The thermodynamic characteristics of vaporization and sublimation of these complexes were determined. The processes of thermal decomposition of the vapour of the compounds in vacuum, hydrogen and oxygen were investigated by using mass spectrometry in the temperature range 170–550‡C for Ru(aa)3 and 150–620‡C for Ru(tfa)3. The threshold temperatures of the stability of the vapour of the complexes and the rate constants of the thermolysis processes were determined. The main gaseous products of the thermal decomposition and the dependences of their composition on the presence of hydrogen and oxygen were established.