A process for synthesizing a Mn4+ doped phosphor includes contacting a precursor of formula I, Ax[MFy]:Mn4+   I at any temperature in a range from about 200° C. to about 700° C. with a fluorine-containing oxidizing agent in gaseous form; maintaining the temperature during a contact period of at least one hour; and, after the contact period, reducing the temperature at a rate of ≦5° C. per minute; 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 the absolute value of the charge of the [MFy] ion; y is 5, 6 or 7.