The vapor pressures and heats of sublimation of ruthenium and osmium have been measured using a microbalance technique based on the Langmuir method. Heats of sublimation at 298 °K were calculated with the aid of free energy functions. Least squares lines for the vapor pressure data, heats of sublimation, and normal boiling points were obtained as follows : Ruthenium: Log P atm = 7.500 - 32 , 769 T ( 1940 - 2377 ° K ) Δ H s ° ( 298 ) = 156.1 ± 1.5 kcal / mole bp = 4150 ± 100 ° K Osmium: Log P atm = 7.484 - 39 , 880 T ( 2157 - 2592 ° K ) Δ H s ° ( 298 ) = 189.0 ± 1.4 kcal / mole bp = 5300 ± 100 ° K The indicated uncertainties are estimates of the overall limits of error.