A process for preparing a Mn+4 doped phosphor of formula I Ax[MFy]:Mn+4  I includes gradually adding a first solution to a second solution gradually discharging the product liquor from the reactor while volume of the product liquor in the reactor remains constant; wherein A is Li, Na, K, Rb, Cs, or a combination thereof; M is Si, Ge, Sn, Ti, Zr, Al, Ga, In, Sc, 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. The first solution includes a source of M and HF and the second solution includes a source of Mn to a reactor in the presence of a source of A.