Equations were developed to predict the concentrations of anatase and rutile in a TiO2opacified porcelain enamel as a function of the time and temperature of firing. These equations were applicable over the entire useful firing range: from 660° to 940°C and from 1 to 64 min. The constants in these equations yield solubility data as well as the appropriate rates. The solubility versus temperature curve is bimodal. The activation energies are 87.4 kcal/mol for inversion of anatase to rutile, 32.0 kcal/mol for crystallization of anatase, and 28.3 kcal/mol for crystallization of rutile. Copyright © 1969, Wiley Blackwell. All rights reserved