A process for preparing a Mn4+ doped phosphor of formula I Ax[MFy]:Mn+4  I includes contacting a mixture of a compound of formula Ax[MFy], a compound of formula AX, and a Mn+n source comprising a fluoromanganese compound, with a fluorine-containing oxidizing agent in gaseous form, at an elevated temperature, to form the Mn4+ doped phosphor; wherein A is Li, Na, K, Rb, Cs, or a combination thereof; M is Si, Ge, Sn, Ti, Zr, Al, Ga, In, Sc, Hf, Y, La, Nb, Ta, Bi, Gd, or a combination thereof; X is F, Cl, Br, I, HF2, or a combination thereof; x is the absolute value of the charge of the [MFy] ion; y is 5, 6 or 7; and n is 2, 3, or 4.