C50Cl10 [S. Y. Xie , Science 304, 699 (2004)] has been synthesized in large quantities enabling the capture of the labile fullerene C-50. In this Communication, we report ab initio calculations on the optical excitation and absorption spectra of C50Cl10. We successfully explain and assign the measured UV-visible absorption spectrum of C50Cl10. The first singlet excitation for C50Cl10 is optically forbidden, and its optical absorption gap is redshifted by 0.6 eV (110 nm) relative to that of C-60. We demonstrate that passivating C-50 with 10 hydrogen atoms and replacing one Cl in C50Cl10 by one methoxy group lead to 100 nm blueshift and 90 nm redshift of the optical gap predicted for C50Cl10, respectively, suggesting C-50 derivatives are suitable for tunable optical applications. (C) 2004 American Institute of Physics.