A process for preparing a Mn+4 doped phosphor of formula I Ax [MFy]:Mn+4 I includes combining a first solution comprising a source of A and a second solution comprising H2MF6 in the presence of a source of Mn, to form the Mn+4 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, 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; and wherein a value of a Hammett acidity function of the first solution is at least −0.9. Particles produced by the process may have a particle size distribution with a D50 particle size of less than 10 μm.