X‐ray diffraction patterns were obtained for a variety of synthetic calcium silicate phosphors containing increasing amounts of lead monoxide, manganous oxide, or both, and fired in different atmospheres and at different temperatures. Powders with and without lead monoxide alone were found to give α or pseudowollastonite structure even when prepared as low as 1150°C.; powders containing manganous oxide, or lead monoxide and manganous oxide as activators showed increasing prominence of the low temperature β structure with increasing content of manganous oxide, even when fired considerably above the “normal” inversion temperature of 1200°C. The effect of steam in the preparation of the phosphors appears to be mostly catalytic with respect to rate of reaction and the development of more perfect crystals. The difference in fluorescent sensitivity between manganese activated phosphors with and without additional lead monoxide can not be explained on the basis of their (identical) lattice structure. The difference in light emission, however, might be correlated with the α and β structures respectively.