We measured fluorescence quantum yields of acetone and 3-pentanone as a pure gas and with nitrogen diluent at room temp. at 20, 507, and 1013 mbar using 248, 266, and 308 nm excitation by calibrating the optical collection system with Rayleigh scattering from nitrogen. At 20 mbar with 308-nm excitation, the fluorescence quantum yields for acetone and 3-pentanone are 7 +- 1 * 10-4 and 1.1 +- 0.2 * 10-3, resp., and each decreases with decreasing excitation wavelength. These directly measured values are significantly lower than earlier ones that were based on a chain of relative measurements. The obsd. pressure and excitation wavelength dependence is in qual. agreement with a previously developed fluorescence quantum yield model, but the abs. nos. disagree. Changing acetone's fluorescence rate const. to 3 * 105 s-1 from its previous value of 8 * 105 s-1 resulted in good agreement between our measurements and the model.