The fluorescence of air was excited by electrons of 4.2 to 43.7 kev and the light intensity If was measured by a 1 P 28 photomultiplier tube. The energy W transferred from the electrons to the gas molecules in a defined space could be accurately measured. The light yield, as defined by Φ = If/W, was experimentally determined as a function of the gas density ρ, or pressure p, respectively. The result is: 1/Φ + p/p′ with p′ = 11.5 mm Hg.