Electric quadrupole transitions of Rb-2 are identified near 540 nm by resonance enhanced two photon ionization (RE2PI) study. The rotational structure shows O, P, Q, R, and S branches, which are quite similar to those of the electric quadrupole transition observed in K-2 [Kim , J. Chem. Phys. 113, 2945 (2000)]. From the high resolution rotational analysis and comparison with the spectra of other alkali diatomic molecules we assign the observed bands to the 1 (1)Delta(g)-X (1)Sigma(g)(+) transitions of Rb-2, which are allowed by electric quadrupole radiation. We obtain molecular constants for the 1 (1)Delta(g) state of Rb-85(2) as omega(e)=49.6652(9) cm(-1), T-e=18 263.960(1) cm(-1), B-e'=0.021 357 9(5) cm(-1), R-e=4.311 71(5) A. (C) 2002 American Institute of Physics.