Extended autoionization structure has been observed in sequential two-photon ionization of Na2 via different vibronic levels of the intermediate B 1Σu state. Partial snalysis of this structure has now been achieved. A dominant Rydberg series (quantum defect δ = 0.4) and a second, weaker series (δ = 0.57) are assigned. Convergence to many different vibronic levels of the ion core is observed, allowing a preliminary determination of ω e and ωexPde for the Na2+ 2Σ+g ground state. A difference of +49 cm-1 between the spectroscopically extrapolated ionization potential and the previously observered ionization threshold is noted and discussed.