This investigation was conducted in an effort to elucidate the mechanism of the strengthening that occurs during processing of a heat-pressed dental ceramic (IPS Empress). The effect of processing on leucite content was determined via quantitative x-ray diffraction. A significant effect of processing was revealed by analysis of variance (P .0001). Post-hoc comparisons by Student-Neuman-Keuls and Duncan's Multiple Range tests revealed no difference in leucite content after perssing, but the leucite content was significantly higher (P .05) after a single surface colorant firing. A second colorant firing produced a further significant increase in leucite content (P .05), but no further increase in leucite content was observed following the remaining colorant and glaze firings. The increase in leucite after firing is consistent with the increase in flexural strength following firing observed by Dong et al in 1992.