A magnetically compensated buoyancy balance is described. This instrument provides precise and continuous indications of gas and vapor densities falling in the range of 0 to 4 g/l. Measurements can be made at temperatures that are freely variable up to 220°C; mildly corrosive vapors can be handled; and the data are automatically corrected for adsorption effects. The long term reproducibility of the readings is 0.001 g/l.Magnetic compensation is attained by adjusting the current in an external electromagnet until this magnet's interaction with a small permanent magnet sealed in the balance beam restores the beam to a standard null position. The virtually linear relationship between this ``restoring'' current and the ambient vapor density is established by calibration with a standard substance. High constancy of calibration has been achieved through the use of a special beam suspension which, though extremely friction free, is unaffected by repeated and relatively severe mechanical and thermal shocks.