The F(1)Pi(1) 'Rydberg' state of Mg-24(16) O has been characterized by two-color Resonance-Enhanced Two-Photon Ionization (R2PI) spectroscopy in the 37000-39000 cm(-1) region. Several rotationally resolved bands, assigned to Mg-24(16)(F(1)Pi(1) <-- X-1 Sigma(+)) vibronic transitions, have been analyzed to yield spectroscopic constants for the F(1)Pi(1) state. Consistent with the ab initio calculations of Peyerimhoff and co-workers, the F(1)Pi(1), state appears to have mixed Rydberg-valence character. (C) 2003 Elsevier B.V. All rights reserved.