We present an ab initio study of the optical absorption and emission spectra of size-expanded nucleic acid base analogues (yA, yT, yT-m, yG, yG-t2, and yC) obtained by benzo homologation (see Krueger, A. T.; Lu, H.; Lee, A. H. F.; Kool, E. T. Acc. Chem. Res. 2007, 40, 141 and references therein). Also examined were the effects of linking to deoxyribose and hydrogen bonding to their natural complementary bases (T, A, C, and G, respectively). The calculated excitation and emission energies are in good agreement with the measured data where experimental results are available. The geometries corresponding to the first excited singlet state of yA and yT are found to be quasi-planar, while those for yG and yC are nonplanar. In general, binding to deoxyribose will red shift the absorbance and fluorescence emission maxima of the y-bases. The ground-state geometries of the Watson-Crick analog base pairs (yAT, yTA, yGC, and yCG) are found to be planar, and the calculated interaction energies are very close to those of natural base pairs, indicating that the y-bases can pair with their natural complementary partners to generate stable base pairs. The base pairing has no significant effects on the fluorescence emission of yA, yC, and yT, but blue shifts the fluorescence emission of yG by 22 run.