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Synthetic photochromic sodalite was prepared (1) by solid-state reaction, (2) from zeolite X-SO2 or zeolite X-Has, and (3) hydrothermally ; the preparations are compared and discussed. X-ray diffraction, ultraviolet and visible light absorption (transmission and reflection), and ESR data served to follow the reactions and characterize the products. The content of sulfur and chlorine and the S :C1 ratio affect the depth of color and the bleaching rate. The partial substitution of other halogens for C1 may speed up or slow down the coloring rate a s a function of electronegativity. Fluorine may shift the visible absorption maximum from 5300 to 5100 A. Stoichiometry and ESR data indicate that the photochromism is effected by the transfer of a n electron between sulfur a s St = and a C1 vacancy between four sodium atoms.

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