The luminescence properties of Eu2+-activated alk.-earth Si-oxynitrides were studied. In the BaO-SiO2-Si3N4 system, a new BaSi2O2N2 compd. was obtained having the monoclinic structure with lattice parameters a 14.070(4), b 7.276(2), c 13.181(3) .ANG., b 107.74(6)°. All MSi2O2-dN2+2/3d:Eu2+ (M = Ca, Sr, Ba) materials can be efficiently excited in the UV to visible region (370-460 nm), making them attractive as conversion phosphors for LED applications. A blue-green emission at 490-500 is obsd. for BaSi2O2N2:Eu2+, yellow emission at 560 nm for CaSi2O2-dN2+2/3d:Eu2+ (d » 0), and a green-yellow emission peaking from 530 to 570 nm for SrSi2O2-dN2+2/3d:Eu2+ (d » 1), the position depending on the exact value of d. BaSi2O2N2:Eu2+ is the most promising conversion phosphor for white-light LEDs due to its high conversion efficiency for blue light from InGaN-based LEDs related to its very small Stokes shift.