The electronic structure of alkaline-earth silicon nitride MSiN2 (M = Sr, Ba) was calculated using the CASTEP code. BaSiN2 is calculated to be an intermediate band gap semiconductor with a direct energy gap of about 2.9 eV, while SrSiN2 is an intermediate band gap semiconductor with an indirect energy gap of about 3.0 eV. As expected, the calculated optical band gaps of MSiN2 (M = Ba, Sr) are lower compared to the experimentally determined values (about 4.1 eV for BaSiN2 and 4.2 eV for SrSiN2). In addition, the luminescence properties of Eu2+ and Ce3+ in MSiN2 (M = Sr, Ba) have been studied. Ba1-xEuxSiN2 (0 <x = 0.1) shows a broad emission band in the wavelength range of 500–750 nm with maxima from about 600 to 630 nm with increaseing Eu2+ concentration, while Sr1-xEuxSiN2 (0 <x = 0.1) shows a broad emission band in the wavelength range of 550–850 nm with maxima from 670 to 685 nm with increasing Eu2+ concentration. The high absorption and strong excitation bands of M1-xEuxSiN2 (0 <x = 0.1; M = Sr, Ba) in the wavelength range of 300–530 m are very favorable properties for application as light-emitting-diode conversion phosphors. Ce3+- and Li+- codoped MSiN2 (M = Sr, Ba) exhibits a broad emission band in the wavelength range of 400–700 nm with a peak center at about 485 nm for BaSiN2 and about 535 nm for SrSiN2. A comparison is made between the luminescence properties of Eu2+ and Ce3+ in the Sr versus Ba compounds. The long-wavelength excitation and emission of Eu2+ and Ce3+ ions in the host of MSiN2 (M = Sr, Ba) are attributed to the effect of a high covalency and a large crystal field splitting on the 5d bands of Eu2+ and Ce3+ in the nitrogen coordination environment.