The absolute vibrational numbering of the Na 2 b 3Π u state has been established by direct observation of the v = 0–13 levels. These b 3Π u levels appear as the lower levels in rotationally resolved fluorescence spectra resulting from OODR excitation of 3Π g , 3Δ g , and 3Σ g + states via b 3Π u ∼ A 1Σ u + mixed intermediate levels. The molecular constants for the Na 2 b 3Π u state are (in cm −1, one standard error in parentheses) T e 13 517.2 (1.3) B e 0.1434 (0.0027) ω e 153.6 (1.1) α e 9×10 −5 (4×10 −4) ω eξ e 0.47 (0.09) r e 3.20 (0.03) Å A e 7.85 (0.60) α A 0.19 (0.16)